Case studies on farm tractors as base machines for single-grip thinnings harvester heads

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1 Case studies on farm tractors as base machines for single-grip thinnings harvester heads J. JOHANSSON Swedish University of Agricultural Sciences, Department of Operational Efficiency, S Garpenberg, Sweden Summary Time and ergonomic studies were carried out for three farm tractor-based single-grip harvesters. The base machines were two models but all attachments varied. One of the machines was also used as forwarder by attaching a wagon and exchanging the harvesting head for a grapple. The studies indicate a productivity at the same level as that of Nordic specialized forest machines (harvesting and forwarding). Ergonomics proved to be fairly good, especially for two of the machines, but still not at the same level as for Nordic specialized forest machines. Mean ground pressure exerted by the farm tractor-based harvesters was at the same level as for some of the Nordic specialized single-grip harvesters which were compared. That was also the case for the highest ground pressure (the highest ground pressure for one wheel with the crane at full length and turned off to the side 90 ). The ability of the machines to operate in the terrain was good. These machine's can also be used for other jobs, such as forwarding, and the traditional variety of jobs in agriculture. Thus the machines function more as attachment carriers than custombuilt specialized machines. With a more careful planning of operations, the machines can be used to a high degree and more effectively. The relatively low investment cost in comparison with that of many custom-built specialized machines often contributes to a reduction in operating costs. Introduction c, c,,, types or work. Farm tractors have been used Conventional Nordic machines for forest har- for a long time as base machines in small scale vesting operations are specialized and can, nor- forest operations e.g. forwarding, skidding, mally, do only one type of work. Furthermore, winching, and processing. The farm tractors the investment level is often high. Standard have been equipped with different kinds of built machines can be interesting alternatives as attachments, simple or advanced, depending on base machines in forestry. They are relatively what operation was to be carried out. Most cheap, compared with custom-built forest attachments have been placed at the rear of the machines, and they can be used for different tractors. C Inmture of Chartered Forester*, 1996 Formry, Vol. 69, No. 3, 19%

2 230 FORESTRY Farm tractors with small single-grip harvesting heads were introduced in forest harvesting operations in the late 1980s, and they were operated mostly in thinning. The number of these machines is so far low. The ergonomic conditions in some of these machines have been improved by more room in the cabin, and seats that can be turned. Farm tractors with attachments for harvesting operations have not become a success, although the number of farm tractors is large. This is mainly because of the relatively poor ergonomic conditions on farm tractors when used as base machines in forestry. The attachments have been placed at the front, over the front axle, on a few machines, which has eliminated the need for the operators to turn backwards. The harvesting equipment used is identical to that used by conventional Nordic harvesting machines. Most concepts (machines) are designed and made by contractors or companies that manufacture attachments and there is usually only one machine of each type. Objectives Studies were conducted with the aim of obtaining an indication of suitability of farm tractorbased single-grip harvesters operating in Sweden. These studies included: fitness for use in harvesting operations, productivity, ergonomics. Material and methods Machines Time and ergonomic studies were carried out for three farm tractor-based single-grip harvesters (Table 1, Figures 1-3). One of the machines was also studied when forwarding after harvesting (machine No. 3). The harvesting head was then exchanged for a grapple and a frame-steared trailer (Table 2, Figure ), made by the operator, was attached to the base machine. Time for detaching the harvesting head and conversion to forwarder (and vice versa) at landing site, normally varied from 15 to 0 min. The exchange of attachment was made easier by using quick-connective couplings. Machines No. 1 and No. 2 were equipped with parallelogram cranes and machine No. 3 was equipped with a knuckle boom crane. In a study of cranes (Johansson Figure 1. Machine No. 1: MB Trac 1000 farm tractor (rigid frame) equipped with Mowi 3865 parallelogram crane and Tapio 250 stroke delimbing harvesting head.

3 FARM TRACTORS AS BASE MACHINES FOR HARVESTERS 231 Figure 2. Machine No. 2: Ford 276 Versatile frame steared farm tractor equipped with Cranab 290 parallelogram crane and Tufab GS 302 harvesting head (rollers). and Myhrman, 1992), parallelogram cranes were considered to be very good in operations where the work tool is moved parallel to the ground surface as in mechanized thinnings and cleanings. Damage to remaining trees was measured, whereby size and location of damage was measured. The type of damage and the cause of the damage was also judged. The Ford harvesting machines were weighed and mean ground pressure was calculated. The highest ground pressure for one wheel was also tested for machine No. 2. The machine was then at no stearing angle and the crane was at Figure 3. Machine No. 3: Ford 276 Versatile frame steared farm traaor equipped with FMV 350 knuckle boom crane and Tufab GS 301 harvesting head (rollers).

4 232 FORESTRY Figure. Hydraulic cylinder for stearing of the trailer, one on each side (machine No. 3). full reach and turned 90 to the side. Mean ground pressure is mass of each machine divided by total contact area between wheels and ground for each machine, and is expressed by the unit Pa. Weighing equipment was Telub 8023, consisting of one central unit and four scale units. The operator on machine No. 1 had less than one year of experience with single-grip harvesters. The operator on machine No. 2 was one of the first operators working on single-grip harvesters and the operator on machine No. 3 had about years of experience on single-grip harvesters. Harvesting methods All machines were working in thinning on a network of strip roads. The thinning area was defined as the and the (Figure 6). The was the area that could be reached by crane from the strip road, the rest of the area was defined as the middle Table 1: Farm tractors and attachments studied Base machine Width Tyres Harvesting Mass Crane type/ Ground (m) head (kg) reach (m) clearance (cm) 1. MB Trac / Ford 276 Versatile /28 3. Ford 276 Versatilet /28 Tapio (approx) Mowi 3865 * strokedel. parallelogram/ 6.7 Tufab GS rollers Tufab GS rollers Cranab 290 parallelogram/ 5.3 FMV 350 knuckle boom/ * Not measured. + After harvesting the harvesting head was exchanged for a grapple and a trailer was attached to the tractor that was afterwards used as a forwarder.

5 FARM TRACTORS AS BASE MACHINES FOR HARVESTERS 233 Table 2: Data of the trailer studied (machine No. 3) Width (cm) Mass (kg) No. of axles Tyres Other information (bogie) 500/ stakes (from Valmet 828) articulated joint (2 stealing cylinders) load gate zone. The machines processed/harvested trees within reach from the place they were standing, work place (Figure 5), then they moved to a new work place. The operators decided which trees to harvest and were instructed to harvest, selected thinning, as evenly as possible across the area. The studies were carried out when the machines were in normal production. Distance between strip roads was approximately 22 m in the study of machine No. 1. First the was harvested, and then the middle zone. The trees in the were felled manually with a chain saw (Figure 6). The trees were felled toward the strip roads so the operator could process the trees from the strip roads. The trees were processed from the top. Normally the operators worked overlapping shifts, which means that the first operator started working (e.g. six o'clock in the morning); three hours later his co-operator started working another 3-hour period with the machine; this period was used by the first operator for felling, other work to be done and breaks; then they swapped jobs again. All trees were in sap when studying machine No. 2. Distance between strip roads was approximately 25 m. The machine first harvested trees in the, from the strip road. Reach could be increased by using natural gaps in the stand beside the strip roads and driving into those gaps. The machine was driven into the between strip roads, after harvesting the trees, where the trees were harvested and the logs were put alongside the strip roads. This harvesting method has been described (including figure) earlier by Gullberg and Johansson (1992), and is shown in Figure 7. Distance between strip roads was approximately 22 m in the study of machine No. 3. This machine was working in the same pattern as machine No. 2. The stands The stand in the study of machine No. 1 was a 25-year-old stand of pine (Table 3). The stands in the studies of the machines No. 2 and No. 3 were year-old mixed stands, being thinned for the first time. Ground conditions (Table ) in the stands were measured according to Terrain Classification System for Forestry Work (Anon. 1992). Figure 5. Work place. From each work place the operators select trees to be harvested. Work area of the cranes is a half circle with the crane reach as radius. Time studies The diameters of all trees in the stands were measured and marked on the trees before the harvesting studies. The heights of sample trees were measured in each study. Time study equipment used in the study of machine No. 1 was a stop watch. The Husky Hunter data

6 23 FORESTRY Figure 6. Harvesting method used for the MB trac machine used. The operator harvests trees within crane reach from strip road. Then the trees in the are felled towards the strip roads with a chain saw and finally the operator, from the strip roads, processes the trees from the top. collector was used in the other studies. The entire work cycle was studied in all studies (including study of forwarding for machine No. 3). most of the logs in the harvesting study (machine No. 3), and some logs harvested earlier, were forwarded after harvesting. The volume of each load forwarded was measured at the landing site. Time study equipment used was the Husky Hunter data collector. Study of ergonomics Ergonomics for machines No. 1 and No. 2 were studied according to An Ergonomic Checklist for Forestry Machinery (Anon., 1990). Machine No. 3 was not studied because the base machine was identical to machine No. 2. The studies were carried out by measuring the machines and interviewing the operators. The checklist contains 13 factors to be measured and/or judged. Each factor can be measured/judged according to a scale of five levels, from very poor to very good. Some of the factors had to be judged subjectively since the checklist is to some extent not a precise instrument. In order to get a better overview, the scale was coded from 1 to 5 where 1 is very poor and 5 is very good. Note that the numbers are not measured values. Vibrations (whole body) were measured in three directions: x (forwards/backwards), y (sidewards), and z (up/down), but were measured only for machine No. 2. Vibrations were measured with the BrUel & Kjasr 2512 and the Brliel & Kjasr 322. Vibrations were analysed using SS-ISO 2631 (Anon., 1982). Noise level was measured with the Bruel & Kjajr 2221.

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8 236 FORESTRY Table 3: Stand data Machine No. of trees per ha Species mix. Pine, spruce, decid., in tenths Mean diameter ± s.d. (cm) Mean volume No. of trees studied Machine no. 1 Machine no. 2 Machine no * * * ,0,0* 10,0,0+ 1,7,2* 1,6,3+ 5,5,0*,6, * ± ± ± ± * Total + Harvested Results Time spent harvesting Time spent processing/harvesting is described by the following model: T = a + b X Vol Expressions used in the models are explained as follows: T = time for the entire work cycle (processing/harvesting), min per tree a = intercept in regression b = regression coefficient Vol = volume per tree, m 3 Intercepts and regression coefficients are significantly different from zero (Table 5, Figure 8). r 1 is low for machine No. 1. No significant difference was found between tree species. Predicted times for the machines No. 2 and No. 3 are a little lower than for machine No. 1. However, this is normally the case when using roller fed harvesting heads instead of stroke delimbing harvesting heads, especially if the trees are large. In addition, the trees were felled by the machines (No. 2 and No. 3) instead of a chain saw. Time for harvesting small trees is larger in the than in the middle zone for machine No. 3, but time is the opposite for harvesting larger trees; probably one major factor affecting this is that there was less space to handle large trees in the than in the. Time for this harvester is a little lower than for machine No. 2, probably depending on during what period of the year the respective study was carried out, machine No. 2 was studied when the trees were in sap. Mean time for harvesting, min tree" 1, was less for machine No. 1 than for the Ford machines (Table 6), but mean diameter was less also Table : Ground conditions in the studies (l=best and 5=poorest) Machine Season Bearing capacity* Ground conditions Surface structure Slope Machine No. 1 Machine No. 2 Machine No. 3 Summer unfrozen soil Summer, unfrozen soil Winter, frozen soil 10 0 cm snow 1 1 t 1 65% = 3 35% = * Called 'ground conditions' in Terrain Classification System for Forestry Work (Anon. 1992) + Only for harvesting during winter with frozen soil

9 FARM TRACTORS AS BASE MACHINES FOR HARVESTERS S MB Trac 1000/Tapk) 250 ""~ ~~ MB Trac 1000/Taplo 250 Ford 276 Versatlle/Tiifab QS 302 Ford 276 Versatile/Tuteb QS Ford 276 VersaMle/Tufab QS 301 Ford 276 Versatlle/Tufab GS Figure 8. Time models for the machines studied. Tree volume (m 3 ) (Table 3). Moving time per tree was less for machine No. 1 also, probably depending on longer reach and no driving in gaps or between strip roads. But, it should be noted that the middle zone trees (machine No. 1) were felled manually with a chain saw which accounts for extra costs. Productivity in harvesting Productivity for the studied farm tractor-based single-grip harvesters (ground conditions in Table 3) varied from 86 to 106 trees per effective hour in the and from 68 to 93 trees per effective hour in the (Table 7). Time for moving between work places accounted for 10.8 to 26.0 per cent of Table 5: Intercepts, regression coefficients, and r 2 in the regression analyses Machine Intercept in regression Regression coefficient r 2 Diameter interval (cm) Machine No. 1 Machine No. 2 Machine No

10 238 FORESTRY Table 6: Mean timein thinning Machine Time per tree, mean ±s.d. (min) Processing/harvesting* Reaching out Moving No. of trees per work place Machine No. 1 Machine No. 2 Machine No ± ± ± ± ± ± ± ± ± ± ± * Reaching out excluded. total effective time. The highest percentage of moving time was for the Ford harvesters. Those machines were operating in the stands between the strip roads also, which slowed down moving speed. Working on the strip road they also used natural gaps in the stands which caused more moving time, but reach was increased. Studies of other farm tractor-based singlegrip harvesters in thinning (Ryyniinen, 199) also indicate a high productivity. At the breast height diameter of e.g. 11 cm productivity for Fendt 380/Keto 51 varied from approximately 90 to 110 trees per effective hour, and for Fendt 380/Patu 350 from approximately 160 to 190 trees per effective hour. In these two cases the cranes were attached to the base machines at front, over the front axle. Productivity in a study of medium sized single grip harvesters in thinning (Hellstrom, 1987), varied from approximately 85 to 115 trees per effective hour for three of the harvesters (Table 7). Mean diameter of removals was 10.1 cm. Time for moving between work placed varied from approximately 13.5 to 16.7 per cent of total effective time. Ground conditions (G, Y, L) according to Terrain Classification System for Forestry Work (Anon., 1992) were 2, 2, 2 and the stand was spruce dominated. Total productivity (MB Trac and Ford machines) was at the same level as that of the Nordic single-grip harvesters. Time for forwarding Logs forwarded were saw logs and pulp wood. A few logs of special sawn timber were also forwarded. The operator tried to sort each load if possible. However, since the loads often contained some logs of the 'wrong' assortment range, extra driving had to be done at landing site when unloading. Mean volume of loads was 9.2 m 3 and 8.5 m 3 for saw logs and pulpwood respectively (Table 8). Other data (Table 8) also indicate that forwarding of pulp wood requires more work than forwarding of saw logs, except for unloading. Mean time was 5.33 min m~ 3 and 6.0 min m~ 3 for saw logs and pulp wood respectively (Table 9). Most time concerns loading, especially for pulp wood, probably because pulp wood logs are much smaller than saw logs. Driving distance from/to landing site and when loading is very important also. Mean volume per load in forwarding was 6., 6.5, and 6.2 m 3 respectively in one study of the three forwarders Gremo, Bruunett Mini, and Norcar in thinning (Ericsson et al., 1987). Productivity for loading was 18.1, 19.1, and 15.7 m 3 per effective hour respectively. Productivity when unloading was 8.5, 100.0, and 76.0 m 3 per effective hour respectively. Driving speed without load for the three forwarders was per cent lower than for the farm tractor-based forwarder. When loaded driving speed was 3 5 per cent lower.

11 FARM TRACTORS AS BASE MACHINES FOR HARVESTERS 239 Table 7: Productivity, farm tractor-based single-grip harvesters compared with three Nordic single-grip harvesters in thinning Machine Productivity (trees/m 3 per eff. hour) Mean diameter, ±s.d. (cm) Mean vol. (m 3 ) Reach (m) Moving (% of eff. time) Machine No. 1 Machine No. 2 Machine No. 3 Valmet 901 and * no Rottne Rapid and * Bruun Two also working off strip road 106/.63 93/ / / / / / / / / ± ± ± ± ± ± (approx.) 13.5 (approx.) 16.7 (approx.) 16. (approx.) * Motor manual felling of trees. Ergonomics Ergonomic factors for the farm tractor-based logging machines were, in total, judged to be fairly good, but still not at the same level as that for the two wheeled Nordic single-grip harvesters FMG 250 E and Skogsjan/LL 87 (Gellerstedt, 1989; 1990). Some factors were at about the same level and some factors were poorer. For both machines factors to be improved were especially 'entering/exiting the machine' and 'level of noise', both considered to be very important (Table 10). Poorest conditions were measured/judged for the MB Trac machine, where e.g. the factors view and work position were not acceptable. Machine No. 1. The lowest rating was given to the factors 'view' and 'entering/exiting', and these factors were not acceptable. The windows Table 8: Forwarding: data influencing time. Machine No. 3 Volume per load, m 3 Average length of load, m No. of work places per load when loading No. of crane-cycles per load (loading) No. of crane-cycles per load (unloading) Volume, m 3 per crane-cycle (loading) Volume, m 3 per crane-cycle (unloading) Volume, m 3 per work place (loading) Driving distance when loading, m Driving distance without load, m Driving distance with load, m Saw logs (7 loads) Pulp wood (5 loads)

12 20 FORESTRY Table 9: Time (min rrr 3 ) and productivity (m 3 eff. hour" 1 ) for forwarding. Machine No. 3 Operation Loading (incl. moving when loading) Driving with and without load Unloading Total forwarding time/productivity Saw logs, time/productivity 3.21/ / 0.90/ /11.2 Pulp wood, time/productivity.30/ / 0.82/ /9. were standard model for agriculture and were not protective against hard objects. They were also too small, which caused poor view and by reason of that rather poor work position. It was difficult to enter/exit the machine because the lowest step on the ladder was too high, and there was no handle to hold on to. Work position was judged to be poor, mainly affected by the poor view. The operator's seat was a standard seat for agricultural conditions and was not exchanged for a 'forest seat'. The seat was not as stable as a seat for forest conditions and was judged in the middle of the scale used. The factors 'cabin', 'cabin climate', 'noise' (considered to be very important), and 'maintenance' were judged second best. There was not enough room for personal belongings, and there were some hard edges which could hurt the operator. Furthermore, cabin climate could be too warm in the summertime. Hearing impairment is considered likely if the equivalent level of noise during a typical work day exceeds 85dB(A). Noise level in this machine (one side window was broken) was a little more than 76 db(a) inside the cabin, thus far below the level at which hearing impairment may occur. However, the level of noise was higher than the maximum level of 75 db(a) recommended in the checklist. The operator had to do some climbing for maintenance of the machine. The factors 'levers', 'instruments', and 'exhaust gas and dust' were given the highest rating. The factors 'lighting', and 'vibrations' were not measured/judged. Lighting was not Table 10: Ergonomic check-up (1 = very poor and 5 = very good) Ergonomic factor Machine Machine No. 1 Machine No. 2 Entering/exiting Work position Cabin Operator's seat Levers Instruments Climate in cabin View Lighting Noise Exhaust gas and dust Vibrations Maintenance 2 2 3* t 5 3* 5 5 t * Standard chair for agriculture conditions, not exchanged for forestry conditions. + Not measured, work only during day time. * Not measured.

13 FARM TRACTORS AS BASE MACHINES FOR HARVESTERS 21 Table 11: Calculated ground pressure for the farm tractor-based harvesters studied and some Nordic conventional single-grip harvesters Machine Mass (kg) Reach (m) Ground Mean pressure (kpa) Highest Machine No. 2 Machine No. 3 FMG 070 Ponsse HS 15e* t * Bogie on rear axle. t Pivot axle locked. 98 kpa with pivot axle not locked. measured since the operator was working only day time. Vibrations were not measured, but (according to the operator) the cabin swayed quite a bit during operation. Machine No. 2. Ergonomics for this machine was rated to be better than for machine No. 1 (Table 8). The lowest rating was measured for the noise factor (76-78 db(a)), below the level at which hearing impairment may occur but higher than the maximum level recommended in the checklist. The noise factor was rated medium class. Noise was mainly caused by the hydraulics under the floor. That level could be reduced by a noise isolating carpet on the floor. The operator's seat was a standard seat for agricultural conditions and was not exchanged for a 'forest seat'. The scat was not as stable as a seat for forest conditions and was judged in the middle of the scale used. Most other factors were judged second best with some minor remarks. Entering/exiting the machine was easy except for the lowest step on the ladder. The step was too high. Work position was fairly good but the operators seat could not be adjusted for height. The articulated frame steering made it often possible for the operator to get the trees in front of him which lowered the strain on his neck. The cabin was slightly narrow according to the checklist. Cabin climate could be too warm summertime. The view out was good, especially close to the machine. The view upwards was limited, which sometimes made it difficult to select trees to be harvested. The ease of maintenance was rated as good, but there was not much room to keep maintenance equipment. The factors 'levers', 'instruments', 'exhaust gas and dust', and 'vibrations' were given the highest rating. Measurements of vibrations showed that the operator could be exposed to vibrations at least one 8-hour shift without exceeding the limit for fatigue and lowered capacity for work (regarding vibrations). The factor 'lighting' was not measured/ judged because harvesting was done only during the day time with no need for extra lighting. The overall impression was that ergonomics for machine No. 1 could be rated in the middle of the scale used. Some factors were good or fairly good. Other factors were poorer e.g. such important factors as 'entering/exiting', 'view (work position)', 'noise', and 'vibrations'. Ergonomics for machine No. 2 were a little better, but should be improved especially regarding level of noise. Ergonomics for the machines No. 1 and No. 2 were similar to ergonomics for 10 farm tractors studied by Hansson et al. (1990). The biggest difference concerned space in the cabins which was better for machines No. 1 and No. 2, and view for machine No. 2. These two factors had a positive influence on work position (machine No. 2) which was judged as fairly good. Using farm tractors in forestry increases importance of good work position. Damage to remaining trees The tendency to cause damage to the remaining trees in harvesting was similar for the three

14 22 FORESTRY machines. Most of the damaged trees (75 80 per cent) were damaged when processing. Only bark was damaged. No damage was larger than 100 cm 2 for the machines No. 1 and No. 3. Machine No. 2 was working when the trees were in sap, which is probably one reason why that machine caused some damage larger than 100 cm 2. The main part of the damage was located above stump. The frequency of damage to remaining trees was per cent, 8.8 per cent, and 5.6 per cent for the machines No. 1, No. 2, and No. 3 respectively. Forwarding (machine No. 3) damaged another 2.5 per cent of remaining trees. Size of damage was less than 100 cm 2 for all trees, and most damage was located above stump. Most damage was caused during crane work. Total frequency of damaged trees was 8.1 per cent (harvesting and forwarding) using this machine. Froding (1992) found that the damage to remaining trees in thinning was mainly located between the stump and up to 1.5 m height on the stems when harvesting with conventional single-grip harvesters. When harvesting with single-grip harvesters 5.9 per cent of remaining trees were damaged (trees damaged when forwarding were included). When loading 0.2 per cent of remaining trees were damaged, and 2.2 per cent of remaining trees were damaged when driving (harvesting and forwarding). Cause of damage was not identified for 0.2 per cent. Cost The price for a farm tractor-based single-grip harvester is approximately SKr 1 million of which the harvester is SKr The price for a custom-built wheeled single-grip harvester of the same size is approximately SKr 1.5 million. So far farm tractor-based single-grip harvesters are new and not fully tested over a lifetime for the machines. We do not know exactly about e.g. productivity, repairs, fuel consumption, and reliability in the long term. However, repair cost for attachments on farm tractors is most likely the same as for custombuilt machines as the attachments are identical. Furthermore, there is no indication so far that repair costs for standardized base machines would be higher than for custom-built machines. Wages for both types of machines are the same also. For a cost estimate the following can be assumed for both farm tractor-based and custom-built single-grip harvesters: Total use base machine and 8 years, crane eff. hours harvesting head years, 6000 eff. hours Interest 9 per cent Depreciation base machine and 80 per cent of crane investment harvesting head 90 per cent of investment Repairs and maintenance SKr 165 per eff. hour Fuel, lubr., blades, SKr 60 per eff. chains, and various hour costs Wages SKr 20 per eff. hour Total machine cost, according to assumptions, is SKr 592 and SKr 652 per effective hour for farm tractor-based and custom-built singlegrip harvesters respectively. Harvesting cost for the machines in the studies varied widely, from SKr per m 3 (machine No. 1, ) to SKr per m 3 (machine No. 3, middle zone). The highest cost was in the stand with the smallest trees. Harvesting cost for the custom-built machines compared was in the range of SKr per m 3 (small trees). For manual felling of trees with a chain saw (machine No. 1) an additional cost will occur. When using overlapping shifts the operator on the ground is used for felling those trees. According to Olsson (1986) productivity for felling the trees in the study of machine No. 1 is approximately 3.15 m 3 per hour (bid. delays < 15 min.). The total additional cost for felling trees, including costs for the chain saw, is approximately SKr 83 per m 3. Discussion All test variables were tested in the regression analyses, individually or combined. The models shown are simple but easy to use and understand. r 2 for the MB Trac machine is low; prob-

15 FARM TRACTORS AS BASE MACHINES FOR HARVESTERS 23 ably one major factor affecting this was that the operator in this machine was less experienced in harvesting operations than the other operators. The 'operator' factor is, however, difficult to measure, r 2 can probably be higher with help of more observations. There are probably other factors which also can help to explain time, such as geometric distribution of the trees, number of trees/ha, and thinning grade. One of the most important factors is probably the 'operator' factor. Disturbances during some part of the work cycle, in harvesting operations, often lead to a large deviation from expected time when production is not disturbed. It could therefore be of interest to give special attention to extreme values in future studies as well as variation in stand conditions. Differences between farm tractor-based single-grip harvesters and conventional Nordic single-grip harvesters are a function of the advantages and problems that occur with the farm tractors as base machines. A standardized base machine which can function as carrier for attachments for different types of work increases machine utilization and flexibility. One disadvantage is the increased demands on the operators to be skilled in the different types of work. Terrain mobility was fairly good, especially for the two Ford machines with articulated frame stearing. A short wheel base contributed to a good mobility in the terrain, and so did a good ground clearance of the Ford machines. Mean ground pressures are calculated according to ideal conditions. That means that the machines were standing still and all wheels were in total contact with the ground. It should be noted that ground pressure is dependent on several factors. These factors can vary between machines of the same make, e.g. length of crane, mass of attachments, extra mass due to modification, and wheels. Ground pressures shown for the studied machines are valid for those, and for machines with identical attachments and modifications. Mean ground pressure for two wheeled single-grip harvesters was 51 and 8 kpa (Myhrman et al., 1990; Granlund et al., 1992). The highest ground pressure for one wheel was about twice as high compared with mean ground pressure (including one of the farm tractor-based harvesters). Mass of the conventional harvesters was.9 and 1.95 tonne respectively. Mean ground pressure for the studied farm tractor-based harvesters was at the same level as that of the conventional Nordic single-grip harvesters compared (Table 11). Wasterlund (1992) considers that machines with a weight of 30 tonnes should be avoided on many sites in order to reduce soil compaction in depth. And he continues: 'A 5 10 tonnes machine with good tyres and rather low inflation pressure could come down to a real ground pressure of kpa which may be an acceptable level of soil disturbance'. Ground pressure during work also varies due to dynamic effects. Transportation cost for farm tractor-based logging machines to the next logging site, is the same as for purpose-built logging machines, if they are used only for one operation. If they are used for other operations at the same time, the total transportation cost will be reduced. At a harvesting volume of about 500 m 3 per site the transportation cost is at least SKr These base machines then function as attachment carriers instead of specialized machines. Frequency of damaged trees when harvesting and forwarding with farm tractor-based machines was a little higher compared with Nordic conventional forest machines. One reason was probably that these operators to some extent were less experienced in harvesting operations. Another reason was probably that one machine was operating when the trees were in sap. Other factors of importance for frequency of damage were probably factors such as stand factors, ground conditions, and machine factors. Location and cause of damage were about the same for conventional Nordic single-grip harvesters and farm tractor-based single-grip harvesters. The farm tractor-based machines were mostly moving on harvesting residues and hence very little impact on ground surface was observed. Finally, the level of investment is quite low. Thus, the conclusion is that the machines studied had a good productivity, fairly good ergonomics and a low cost level. In the long term a forest company can, with the right planning, obtain a high yearly machine use.

16 2 FORESTRY Acknowledgement These studies were financed by the project 'Private Forestry'. References Anon SS-SIO Vibration and Shock - Guide for the Evaluation of Human Exposure to Whole-body Vibration. Svenska Elektrislca Commissionen. 16 pp. Anon An Ergonomic Checklist for Forestry Machinery. The National Institute of Occupational Health, The Forest Operations Institute of Sweden and Swedish University of Agricultural Sciences, Kista. Manual, 3 pp. Anon Terrain Classification System for Forestry Work. Forest Operations Institute of Sweden, Kista. Manual, 28 pp. Anon. 199 Forestry Vocabulary. TNC 96. Sveriges skogsvardsforbund & Tekniska Nomenklaturcentralen, Solna. Ericsson, M., Myrman, D. and Eickhoff, K Gallringsskotare Forskningsstiftelsen Skogsarbeten, Spanga, Sweden. Froding, A Thinning damage to coniferous stands in Sweden. Swedish University of Agricultural Sciences, Department of Operational Efficiency, Garpenberg. Thesis. Unpublished, 71 pp. (In Swedish with English summary.) Gellerstedt, S Operating Conditions in Forestry Machines. An Examination of the Single Grip Harvester FMG 250 E. Swedish University of Agricultural Sciences, Department of Operational Efficiency, Garpenberg. Uppsatser och Resultat nr 158, 5 pp. (In Swedish with English summary.) Gcllerstedt, S Operating Conditions in Forestry Machines. The Single Grip Harvester Skogsjan/LL 87. Swedish University of Agricultural Sciences, Department of Operational Efficiency, Garpenberg. Research Notes No 189, 22 pp. (In Swedish with English summary.) Granlund, P., Karlsson, L., Landstrom, M. and Myhrman, D Skogsarbeten testar engreppsskdrdaren Ponsse HS lse. Forskningsstiftelsen Skogsarbeten, Kista. Resultat nr 3. Gullberg, T. and Johansson, J Farm tractor based single grip harvesters. University of Agricultural Sciences, Department of Forest Extension, Garpenberg. Small Scale Forestry 2, 9-1. Hansson, J-E., Adolfsson, K., Kilberg, S. and Larsson, L Jordbrukstraktorn som arbetsplats. National Institute of Occupational Health, Stockholm. Undersokningsrapport No 15. ISRN AI/UND 90/15 SE. Hellstrom, C Bestdndsgaende eller stickvagsgiende engreppsskbrdare i ffrstagallring. Forskningsstiftelsen Skogsarbetern, Spanga. Resultat nr 16, pp. Johansson, A. and Myhrman, D Parallellstyming-ett enkelt sittt att kombinera vikarmskranens och teleskopkranens fordelar. SkogForsk, Kista. Resultat nr 11, pp. Myhrman, D., Granlund, P. and Gardh, R Skogsarbeten testar, FMG 070. Forskningsstiftelsen Skogsarbeten, Kista. Resultat nr 10. Olsson, P Stand Treatment - Economic Analyses of Technical and Biological Factors. The Forest Operations Institute of Sweden, Spinga. Report No. 6. Ryyninen, S. 199 Farm Tractor Harvester in First Thinning of Pine. Work Efficiency Institute, Helsinki. TyStehoseuran julkaisuja pp. (In Finnish with English summary.) Wasterlund, I Extent and causes of site damage due to forestry traffic. Scand. J. For. Res. 7, Received 2 May 1995