Title: Backside story

Pages: 80 - 81


Author: Phillip Hill

(Captions page 80) The most dangerous frequencies for spine deformation are 4-8Hz. The graph above shows the dampening action achieved using the Bostrom XH suspension unit. Magnification of low frequency vibrations is unimportant in terms of the human body.
Height control inset 1. gives seven ergonomic positions over a range of 60mm. The ratchet handle inset 2. left,
adjusts for driver's weight by pre-loading a torsion bar. The indicator right helps the driver adjust this mechanism so that he normally sits in the middle of the suspension stroke

Nearly three-quarters of all tractor drivers aged 20-29 suffer from spinal deformation. This discovery of the early 'sixties has had a profound effect on tractor seat and suspension design thinking.
Worker protection boards in several European countries have insisted that seats fitted to tractors should come up to standards recommended by the various institutes. Foremost among these is the National Testing Institute of Sweden which, according to tractor manufacturers, is one of three which now effectively dictate the design of tractors.
Volvo, daunted perhaps by the strict NTI regulations, took its tractor problem to UOP Bostrom of Northampton. Bostrom gave Volvo the Argus seat and XH suspension unit, a 1974 Design Council Automotive Award winner. Without this request from Volvo, Bostrom would probably not have undertaken a project like this, says technical director Dr Cedric Ashley. Now, with an anticipated production rate of 20,000 per year, the seat and suspension unit could become one of Bostrom's biggest sellers.
The project was very much design-by-regulations. Each country has its own set of standards and it is likely that the EEC will adopt common standards in due course. Testing procedures and desired performance characteristics have to be thrashed out in committee: Bostrom as a company is in an ideal position, having stronger representation on important committees than any other single company in Europe. Cedric Ashley is a member of the committee working out the draft British Standard on tractor vibration, and George Lowe, technical manager, is helping to formulate an ISO draft on seat specification and performance.
Tractor manufacturers, by and large, do not touch seats or suspensions: it is all subcontracted to specialists. One of Bostrom's basic criteria in designing the XH suspension was that it should be compact enough to fit onto any tractor without costly modifications. Job designer Bob Lacey succeeded by using a fully collapsing 'X' mechanism which gives the lowest profile (37mm) of any known suspension system. When in operation, the height of the seat (for good ergonomics) can be adjusted by up to 60mm giving a suspension stroke of between 100mm and 160mm. This is achieved by a seven position control.
To make sure that drivers of differing weights have their mid-ride position at the middle of the suspension stroke, a ratchet handle preloads a torsion bar, the basis of the whole suspension. By increasing the tension on the torsion bar, the suspension will stiffen for the heavier man, and vice versa for a light driver. An indicator shows the driver at what position in the stroke he is sitting. The unit is fitted with a double-acting hydraulic damper to absorb vibrational energy, and for violent motion not normally encountered, a rubber bump stop prevents any damage.
Bostrom is one of few companies using a torsion bar for the suspension. Most other manufacturers use coil springs or, on cheaper seats, simply rubber. The advantage of a torsion bar is that for a given stiffness in the suspension, the bar can cope with a far wider range of loads: in Bostrom's case from l5kg to 116kg. Alternatively, for the same load, the torsion bar system would give a softer ride than its coil-spring equivalent.
The side arms, in the form of an X, connect to the top and base plates through steel rollers and sintered phosphor-bronze bearings: this gives long life to the moving parts and silent operation.
The Argus seat, while not as critically engineered as the XH suspension, was designed principally with the comfort and safety of the driver in mind. Many tasks of the driver require him to sit sideways in the tractor seat looking back towards his implements, so moulded polyurethane foam was used as the seating material, with a pvc skin. The soft roll-over pad at the top is particularly padded. All edges of the seat are protected by rolled plastics-coated beading. The seatpan itself is made of rigid pressed steel.
One of the big problems of tractor seats till now has been that, especially in hot weather, great discomfort has been caused because of bad air circulation between the driver and the seat. Unfortunately, good air flow is incompatible with a waterproof finish so the designers dealt with the problem by using a detachable cloth-covered pad insert, recessed into the seat. When the tractor is not in use, the cushion can be taken undercover if rain threatens. Another comfort feature is the deep ventilation channels which promote the circulation of air. The arm-rest has been designed to withstand abuse: it will support 110kg (a 17st man) at any point along its length if the weight is static.
A novel feature of the seat is the optional fitting of a seat belt, operated single-handed. About twenty people on average are killed in this country each year through roll-over accidents. Seat belts on tractors is a requirement in the United States and is soon likely to be enforced in the UK; a safety cage is already compulsory, and many tractor manufacturers are plumping for a complete cab.
At a price to the tractor manufacturer of 8.10 for the seat and 14.17 for the suspension, the system should prove very popular. Its added appeal is that, with one exception, it can be easily fitted to old tractors.



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