elnit0 bics by Paul Brockington, MIMechE
Page 50
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Tanker design problems
THE WORK HARDENING of aluminium by continuous high-frequency vibration when a tanker is running light is a major problem for the operators of light-alloy tankers according to The Yorkshire Engineering and Welding Co Ltd of Bradford. In the case of a 24-ton articulated outfit the maximum capacity of an aluminium petroleum-spirit tanker is typically 4647ga1 whereas the load of a comparable steel tanker is 4180gal. The extra £1000 that has to be paid for an aluminium tanker is, therefore, relatively soon offset by the increased carrying capacity.
But the fatigue cracks that tend to develop in the partitions of an aluminium tank as the result of high-frequency vibrations can add considerably to down-time as well as repair costs. To reduce the incidence of fatigue cracks of aluminium tankers to a minimum, Yewco insists on equipping an articulated unit with air suspension or, as a second choice, solidrubber suspension, which give a softer ride when the vehicle is running light. In the case of steel tankers multi-leaf springs are considered suitable; single-leaf springs are not considered satisfactory for tanker applications owing to the risk involved in the event of fracture.
It is pertinent to add that the improvement in roll stability provided by Dunlop Stabalair and Hands Neway wide-track suspensions and other up-to-date systems (compared with the inferior stability of older types of suspension) is cited by Yewco as a development of major importance to tanker makers. The difficulty in obtaining good roll stability is increased when single tyres are fitted, owing to the covers' greater flexibility. In the case of tankers made by the company, stability has also been improved since 1966 by reducing the overall height of the superstructure by approximately 12in. which has given a reduction in the centre of gravity of the vehicle when laden of some 6 /7in. Height reduction has been made possible by employing tanks with throated end sections.
Home Office approval Despite the benefits of improved suspension, the problem of work hardening of aluminium is still a major one and the prospect of Home Office approval of grp tanks for the transport of petroleum-spirit gives promise of a workable solution. A grp spirit tank is being developed by Yewco for fitting to a 24-ton-gross artic in conjunction with Prodo rite Ltd of Wednesbury, Staffs, and has been licensed by the Home Office for evaluation trials over a period of 12 months. It will be equipped with a 10-ton axle, Dunlop Stabaiair suspension and Michelin super-single 18 X 19.5 tyres.
The use of plastics tankers has not been permitted in the UK for carrying petroleum-spirit as it is difficult to safeguard the loads against static electricity. The Home Office has recently approved a method of dealing with static which comprises bonding the footvalve to an external earth. Apparently this is a completely satisfactory solution and affords an alternative to incorporating wire mesh in the grp shell to provide an earth, which would complicate manufacture and add to its cost. The weight of the tank is approximately the same as that of a comparable aluminium type.
An increase in maximum gvw to more than 32 tons would not, in Yewco's opinion, benefit the operators of petroleum-spirit or fuel-oil tankers because the majority of filling stations cannot accept more than the 6500/6600ga1 of fuel that can currently be carried by a maximum-load articulated tanker. Yewco technicians point out that exploiting the potential of the new regulations to the full, as they apply to 32-ton-gross artics, would involve reducing the wheelbase of a two-axle tractive unit of a four-axle artic and that this would necessitate moving the kingpin forward a considerable amount relative to the drive axle. But this would overload a 6-ton tractive unit front axle, and a tractive unit with a 7-ton axle is currently unobtainable, despite the fact that an extra ton on the front axle is now permitted by the regulations.
, Moreover, obtaining adequate braking would be a major problem unless a 7-ton axle of sufficient beam strength and with suitable suspension were available.
While Yewco is "having a look" at three-axle tractive units for gvws up to 38 tons based on existing-design parameters, the long wheelbase of the machine would increase the turning circle to an extent that would not be acceptable to many operators. They observe that operators carrying containers need the 11-ton axle to accommodate 40ft containers and that providing adequate braking on larger vehicles would be the major problem, as in the case of the shorter-wheelbase 32-ton attics, although good vehicle handling could readily be obtained. Employing an 11-ton axle would involve a loss of payload on account of the heavier tyres, suspension and so on that would be required.
A type of mounting that allows the vehicle chassis to flex independently of the shell is regarded as essential by Yewco in the case of aluminium tanks to obviate high localized stressing of the shell structure.
Changes in the axle-spread regulations that have enabled three-axle artics and rigids to be operated at gvws up to 24 tons has given vehicles in this category the status of the "most useful tool" for tanker operators, in the Yewco view, because of the greater latitude provided with regard to weight distribution compared with a 32-ton-gross artic;and in many cases its improved manoeuvrability in restricted areas is in its favour operationally. According to Yewco the proportion of 24-ton-gross attics to rigids of the same rating is currently 75 /25; it is anticipated that in due course it will be 50/50.
The advent of the axle-spread regulations has been a boon to operators seeking to extend the useful life of a tank economically. For example it is now expedient to remove the 4000gal mild-steel tank of an old 24-ton eight-wheeler, rebuild it to increase its capacity to 5700gal and fit it with running gear for operation at 32 tons gross.