2. Materials, methods and results

TESTOPS final report

3

2. Materials, methods and results 2.1 Definition of test motions (Work Package 1) The objective of work package 1 was to measure the characteristics of representative motions that may cause seat suspensions to exceed their range of free travel in four different types of off-road test vehicle. Acceleration time histories were recorded at the base of the seats so as to establish the characteristics of idealised test input motions for reproduction on electro-hydraulic vibrators in the laboratory. 2.1.1 Industrial Trucks Measurements were made on industrial trucks by INRS, using suspension seats prepared and supplied by Grammer AG. 2.1.1.1 Test seats Two Grammer MSG30 (“Movito”) suspension seats (see Figure 2.1.1) were prepared by the manufacturer. The Movito seat is equipped with a mechanical suspension and has a weight adjustment range of 50 to 130 kg. One test seat was prepared with relatively low damping (tension = 103N, compression = 268N) and the other with higher damping (tension = 143N, compression = 356 N). Both suspension seats had a stroke of about 58 mm, and were each equipped with two rubber end-stop buffers mounted on the plate below the suspension. The efficiency of the two seats at reducing the vibration emitted by industrial trucks was evaluated in the laboratory according to pr EN 13490 (1999), classes IT1 (trucks with wheel diameters below 64.5 cm), IT2 (trucks with wheel diameters between 64.5 and 90) and IT3 (trucks with wheel diameters between 90 and 120 cm). Tests were not performed with the class IT4 inputs (seats for all terrain trucks) because the seat suspensions exceeded their range of free travel. Both seats passed the class IT1 and IT2 tests but failed with class IT3. 2.1.1.2 Test vehicle and operators The two suspension seats were successively mounted on a Komatsu FG25 counterbalance truck, with a load capacity of 2.5 tonne, as shown in Figure 2.1.2. The truck was unloaded and equipped with solid tyres. Measurements were made on both seats with three professional drivers. The same heavy operator was used to test both seats (his weight was 83kg, which is within the last 25th percentile for European males). Two different light drivers were used (one for each seat). Their weights were 57.5 (seat with higher damping) and 63 kg (seat with low damping), which are within the first 25th percentile for European males. Care was taken to adjust the seats to their respective weights according to instructions provided by Grammer. 2.1.1.3 Measurement method Vertical accelelerations were simultaneously recorded by three Entran piezoresistive accelerometers on the surface of the seat cushion (this accelerometer was inserted inside a flexible pad conforming to the requirements of ISO 10326-1 (1992), at the top of the suspension mechanism (below the cushion) and at the base of the seat. End stop impacts were monitored by fitting two micro-switches at the level of the two end-stop buffers.


4

Figure 2.1.1 Grammer MSG30 seat prepared Figure 2.1.2 Komatsu 2.5 ton for measuring seat end-stop impacts in counterbalance lift truck used to measure seat end-stop impacts industrial trucks Each seat end-stop impact was analysed separately. Periods of 4s of acceleration time histories around the impact were extracted to establish the caracteristics of typical transient shocks which cause seat end-stop impacts. For each impact and the three levels of measurement (base of the seat, top of suspension, surface of the cushion), the vibration dose values (VDV) as defined by ISO 2631 (1997), were calculated over the 4s periods as well as the corresponding probability density and the power spectral density. The fork lift truck was driven along a length of flat and straight tarmac track with a 25 cm long x 1.80 m wide obstacle placed across it. This method is recommended by prEN 13059 as a standardised test for industrial trucks. The obstacle height ranged between 10 mm and 40 mm according to the tests and was perpendicular to the track. Such obstacles were assumed to be representative of the surfaces on which this type of truck would normally be driven. The driver was asked to maintain constant speed along the test track (10 m before the obstacle and 10 m after). The speed varied between 6 and 12.3 km/h according to the test. The first runs with the first driver were at a speed and with obstacle heights insufficient to generate suspension seat end-stop impacts. However no systematic experiment with progressive increment of speeds or obstacle heights was done to avoid exposing drivers to excessive vibration dose. Speeds above 10 km/h were considered to be unsafe with 30 mm or higher obstacles. 2.1.1.4 Results A total of 24 runs with seat impacts were recorded. With the different acceleration time histories it was possible to distinguish clearly the peaks of acceleration corresponding to the front wheels of the truck rolling over the obstacle and the peaks corresponding to the passage of the rear wheels, with a delay of about 0.6 s at 10 km/h. End-stop impacts were obtained for both seats and different drivers when the truck was rolling over obstacles with a height of at least 30 mm and a speed of at least 6 - 8 km/h. At 10 km/h, the seat may impact twice and even three times (low damping) at 12.3 km/h. Most of the impacts occurred after the rear wheels rolled over the obstacle. Apparently the seat motion due to the front wheels rolling over the obstacle interferes with the motion due to the rear wheel, which gives sufficient energy to generate an end-stop impact. The driver posture was also observed to affect the occurence of an impact.


5

Figure 2.1.3 compares an example of a real acceleration signal recorded at the base of the seat with an idealised signal constituted of an exponential sinus at a frequency of 6Hz (after 1s the peak values correspond to 10% of maximum peak). The equation of this idealised signal is: -2.3t

x(t) = 10 sin 12πt e

This idealised signal simulates only the truck response to the front wheel rolling over the obstacle. The signals do not superpose well because the real signal is a combination for each set of wheels (front and rear) of the shock response and the free response of the truck on its tyres.

Figure 2.1.3 Idealised acceleration signal superposed on an example of the acceleration signal recorded at the base of seat.

2.1.2 Agricultural tractors Measurements were made on agricultural tractors by Grammer AG, in association with their sub-contractor Bundesanstalt für Landtechnik (BLT). The suspension seats used in the measurements were prepared and supplied by Grammer. 2.1.2.1 Test seats The seat used for the agricultural tractor measurements was the Grammer MSG 95G/ 731. The measurements were carried out by BLT in Wieselburg, Austria. The MSG 95 is an air suspension seat with a built-in 12V compressor. The seat has an adjustable shock absorber, an automatic positioning system and a fore and aft isolator that can be locked out. The


6

measured dynamic characteristics of the two supplied seats are shown in Table 2.1.1. Table 2.1.1 Characteristics of the supplied MSG 95 seats. Seat no. 1 Delivered to BLT

Seat no. 2 Delivered to ISVR

125

124

125

122

1,09 m/s2

1,12 m/s2

0,85 m/s2

0,87 m/s2

natural frequency of seat with 40 kg load

1,88 Hz

1,90 Hz

natural frequency of seat with 80 kg load

1,58 Hz

1,60 Hz

Seat data (measured by GRAMMER Test Laboratory) Suspension travel [mm], weight adjustment at highest level Suspension travel [mm], weight adjustment lowest level aws with 56 kg driver, stimulation awb= 1,50 m/s2 aws with 98 kg driver, stimulation awb= 1,50 m/s2

Table 2.1.2 Descriptions of the four test tracks used for the agricultural tractor measurements. Test track

Description of the test track

100 meter bumpy Test track with a length of 100 meters, slats of wood mounted on other track slats. The test track is independent of weather conditions and the basis signal for the measurement is reproducable at any time. 35 meter bumpy track

Test track with a length of 35 meters, slats of wood mounted on other slats. The test track is independent of weather conditions and the basis signal for the measurement is reproducable at any time. The Amplitudes of the basis signal are higher than the basis signals of the 100 meter track

Forestry road

Rough natural road with pot-holes and roots, without road metal and gravel. The average of ascent is about 12 %. The test track depends on the weather conditions and is not reproducable at any time.

New purpose built test track

Slats of wood with a height of 10 cm, fixed on other slats. The length of the test track is about ten meters.

2.1.2.2 Test vehicle and operators The measurements were carried out on a Steyr 9105 tractor on four different tracks, with a range of speeds and a heavy driver. The occurrence of endstop impacts was detected by two micro switches, which were mounted on the base plate and the upper plate of the suspension.


7

2.1.2.3 Measurement method Four test tracks were used in the measurements, as described in Table 2.1.2. For the first set of measurements the adjustment of the shock absorber was set at the lowest level, giving the softest damping of the seat suspension. For the second set of measurements the shock absorber was set to the middle of the five possible positions. 2.1.2.4 Results Figure 2.1.4 shows motion time histories and end-stop impact occurences while driving over the forest road. The greatest number of end-stop impacts, and the greatest difference between the two shock absorber settings, occurred with the new, purpose built, test track. However, this track may not be representative of real operating conditions.

Figure 2.1.4 Motion time-histories and end-stop impact occurences while driving over the forest road with the soft shock absorber adjustment.

2.1.3 Earth mover Measurements were made on a JCB back hoe loader by KAB Seating. KAB also prepared and supplied the suspension seats used in the measurements. 2.1.3.1 Test seats Two KAB 515 seats (see Figure 2.1.5) were prepared for the project. These seats are used in a number of earthmoving machinery applications, including back-hoe loaders. The KAB 515 uses a mechanical suspension in which the weight is supported by a pair of coil springs in tension, disposed horizontally below the top plate of the unit. Motion is constrained by an "X" mechanism, and a telescopic damper is mounted in an inclined position between the


8

lower plate of the unit and a point in the mechanism. Adjustment for the driver's weight is provided by varying the pre-load of the coil springs. Before the field trials, the suspension units were run-in by cycling the units and the dampers separately through 10,000 cycles. The important parameters of the seats are shown in Table 2.1.3. Table 2.1.3 Characteristics of the two suspensions Suspension Serial number

Damper Type

198000088

198000086

Light (160416)

Heavy (78055)

Spring rate N/M (max load)

-1

4.63 Nmm

4.25 Nmm-1

Spring rate N/M (min load)

4.88 Nmm-1

4.56 Nmm-1

Free travel between stops

69 mm

74 mm

Travel including stops

74 mm

90 mm

Damper force at 1.66Hz, 25mm stroke

243 N

1103 N

Damper force at 1.66Hz, 50mm stroke

487 N

N/A

Figure 2.1.5 KAB 515 seat

Figure 2.1.6 Back-hoe loader used for field trials

2.1.3.2 Test vehicle and operators The seats were tested on a JCB model 3CX back-hoe loader (see Figure 2.1.6), which was chosen as being representative of a common class of earth-moving machine of a type known to have ride vibration with relatively large oscillations. Preliminary tests were made using the "Contractor" version, with electric gear change and other options. The full tests were made using a model with manual gearchange, but similar in all respects likely to affect the vibration measurements. Two test drivers took part in each set of tests. For the main, baseline tests, their weights were 66 kg and 90 kg. 2.1.3.3 Test Method Measurements were made using five accelerometers and a displacement transducer, measuring the three translational accelerations on the seat cushion, and the vertical acceleration of the seat mounting plate and the top plate of the suspension, together with the


9

relative displacement of the suspension. The main tests were made using only the three vertical acceleration channels, and the relative displacement, but with the addition of microswitches set to register when top and bottom end-stops were contacted, and when they were penetrated by 80% of the available deflection. A switch was also provided for the driver to indicate at the end of each run whether or not he had sensed an end-stop impact. Test surfaces were therefore chosen to be representative of quarry roads, which had been graded originally with stones on the Macadam principle (not tarmac), with drainage channels across the sloping sections. A track was chosen which had been well used, having developed a range of potholes on the level sections, and still retaining the drainage channels on the slope. For the main tests, a single bump, centred on one of the drainage channels was selected. This was done to enable a higher sampling rate to be used for data acquisition, and to reduce the exposure of the drivers to shock and vibration. A range of speeds was chosen, on the basis of determining how slowly it was necessary to travel without causing overtravel of the suspension with the lighter damping, and increasing the speed incrementally until either the run VDV approached 10 ms-1.75 or the driver felt unhappy about going any faster. 2.1.3.4 Results During the main tests, a total of 62 records were made. The record of a typical data set is shown in Figure 2.1.7.

Figure 2.1.7 Typical data set from the main tests, showing displacement, 3 acceleration traces, and indications of end-stop contact. 2.1.4 Forestry forwarder Measurements were made on a forestry vehicle by NIWL, using suspension seats prepared and supplied by Isringhausen Gmbh. 2.1.4.1 Test Seats Two ISRI 6500/577 air suspension seats were provided, with both “soft” and “hard” damping characteristics (see Figure 2.1.8). The seats had suspension strokes of +/- 50 mm,


10

and provided automatic weight adjustment. Six alternative shock absorbers were provided (W1, W2, W3, H1, H2, H3). The characteristics for these shock absorbers is given in Table 2.1.4. The seats were installed in the forestry vehicle in collaboration with the manufacturer. Table 2.1.4 Data for the six shock absorbers provided for the test seat Damper

Test velocity (cm/s)

Force (pull) (N)

Force (push) (N)

W1 / H1

1,1 1,3 6,6 13,1 1,1 3,3 6,6 13,1 1,1 1,3 6,6 13,1

20 / 45 40 / 110 95 / 280 200 / 610 25 / 45 55 / 130 145 / 350 330 / 820 75 / 130 110 / 335 215 / 715 510 / 1345

33 / 60 70 / 140 135 / 290 300670 50 / 60 110 / 145 210 / 350 470 / 846 105 / 255 180 / 585 300 / 910 385 / 1455

W2 / H2

W3 / H3

2.1.4.2 Test vehicle and operators A Valmet 840 S-2 eight-wheeled forwarder, with an approximate weight of 12500 kg, was chosen for the test vehicle (see Figure 2.1.9). The vehicle was manufactured by PARTEK Forest AB in Umeå, Sweden. Endstop impacts occur more frequently in forwarders compared with other types of forestry vehicles such as harvesters. Measurements were made with two drivers, with weights near the 25th and 75th percentile weights for European males.

Figure 2.1.8 ISRI 6500/577 air suspension seat

Figure 2.1.9 Test vehicle: Valmet 840 S-2 eight-wheeled forwarder at a felling site

2.1.4.3 Test conditions Typical work situations during which endstop impacts occurs were identified. This was done in collaboration with the manufacturer and skilled vehicle operators, in field studies at


11

felling sites. It was found that an endstop impact could occur when a wheel “climbs” up on a stump or a stone and then glides off, or when running over an “edge” (e.g ditchside). It was also concluded that the use of a real test surface for the measurement would cause problems with respect to repeatability and reproducibility. The real surface was found to change significantly for each test run mostly due to the heavy weight of the vehicle. Since the final test requires repititions of many test runs, with different conditions, it was decided to use an artificial, but representative, surface for the measurements. An artificial test surface, which includes a representative and durable obstacle, has been constructed in collaboration with the Swedish National Machinery Testing Institute in Umeå (see Figure 2.1.10).

(cm)

40

40

40

145

Figure 2.1.10 Schematic drawing of obstacle: the arrow indicates the direction from which the vehicle approached the obstacle Vibration measurements were made on the cab floor beneath the suspension seat using Brüel and Kjær (BK) 4368 accelerometers. At the top of the suspension mechanism beneath the seat cushion, acceleration was measured using a BK 4366 accelerometer. Seat acceleration was measured using a tri-axial BK 4322 seat pad. An ‘end-stop indicator’ was fitted to the seat, comprising of a micro-switch that was activated when the suspension system compressed the rubber buffers by about 10 mm at the end of the suspension travel. Signals from the transducers were conditioned using a BK 5974 8-channel charge amplifier and acquired at 400 samples per second into an HVLab data acquisition and analysis system via analogue anti-aliasing filters (Techfilter TF-16). The VDV value for each test run was determined. A test protocol was designed to reproduce end-stop impacts, whilst minimising the vibration exposure of the drivers. For each seat-damper combination, the first test used a vehicle speed of 1.5 km/h. Tests were repeated with speeds increased in increments of 0.5 km/h until an end-stop impact occurred. The vehicle speed was then kept constant until either: • •

3 end stop impacts had been measured, or, 3 measurements had been made without an impact.

Consequently, following the first impact, a maximum of 4 further tests were needed at each speed to satisfy the protocol. If 3 end-stop impacts had been measured, the tests for that seat-damper combination were complete; if 3 measurements had been made without an impact, the tests were continued at the next speed increment. If a speed of 6.5 km/h was reached without producing a set of 3 end-stop impacts, the tests for that seat-damper combination were stopped. 2.1.4.4 Results Figure 2.1.11 shows typical time history recordings for one driver (D1) running over the test obstacle at different speeds until an endstop occurs. The magnitudes of the acceleration


12

peaks can be seen to increase with speed. The graph indicates two impacts at speeds higher than 2.5 km/h. The second impact is generated when the second pairs of wheels climbs of the obstacle. The second peak, produced when the rear wheels leaves the obstacle, becomes more pronounced at higher speeds.

Acceleration,2 m/s

50 40 30 20 10 0 -10 -20 -30 -40 -50

W2 S2.0

50 40 30 20 10 0 -10 -20 -30 -40 -50 50 40 30 20 10 0 -10 -20 -30 -40 -50 50 40 30 20 10 0 -10 -20 -30 -40 -50 1,0

W2 2.5

W2 S3.0

W2 S3.5

EndStop 0,0

0

1

2

3

4

5

Time, s

Figure 2.1.11 Time histories measured on the seat cushion at different driving speeds for driver 1 and shock absorber W2. The bottom graph shows that two endstop events occured at the p highest speed ( ) 16 W1 14

W2 W3

12

H1 H2

VDV seat

10

H3

8

6

4

2

0 0

1

2

3

4

5

6

7

8

VDV floor

Figure 2.1.12 Input VDV versus output VDV for the original ISRI 6500/577 seat for six different shock absorbers. Data are combined for both drivers Input VDV versus output VDV for all test runs and for all 6 dampers is shown in Figure 2.1.12. For these test inputs, the 'hard' dampers ('H' series) showed a better performance than the 'weak' dampers ('W' series).