Designing lorries for heavy haulage
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Scammell has been building 100-tonners since 1919. Graham Montgomerie, takes a close look at one of the leading 'heavies'
'WE ARE sometimes being ac..:used of being conservative and f that means being concerned thout product integrity then, tes, we are . ." That was how )avid Picton, product planning rianager of Scammell Motors, iummed up the company's deign philosophy in the field of leavy haulage.
This company has been in at he heavy end of the market — with the accent on heavy — for • iome considerable time.
The first Scammell 100-tanner was produced in 1919. Thus it was logical to visit the Tolpits _ane factory to discuss the mrious design parameters and their effect on the ultimate product.
The trailer side has not been forgotten and this is where Craven Tasker comes in, but more about this company later.
Scammell is an autonomous profit centre within the Leyland Vehicles group. There are co-ordinated financial targets, personnel and product policies but within these constraints the company operates freely. It does not spend resources overlapping with Leyland but is free to examine anything outside the Leyland product plan.
According to Scamrnell's chief engineer, Jim Moore, the operator wants reliability above all else and this is especially true when police escorts and restricted routes are concerned.
It is difficult to talk about "standard" Scammell vehicles as they tend to be built to individual operator specifications. in the company's experience, the heavy haulage people know their business (although it has been noticed that the expertise gets less as you come down the weight scale) so there are few arguments over specification. If an operator has sufficient reason to want something different, then Scammell will build it.
There are various legal criteria to be considered in the heavy haulage design field — the 150 tons break point for a special order permit is a good example. Scammell, however, has its own design sectors based on the e gineering capability of IF various components, eg mechanical clutch.
Some operators want to kel their options open and still I able to run within Constructic and Use Regulations. The Le land Roadtrain, for example, available as a 50 tons gc machine by means of changes the spring and tyre specific tions and to the fifth-whE mountings. Above this is tl Scammell market.
Scammell has one dividit le in general terms at 130 tons. Os is not a restriction in the :hide design rating as a whole it a Scammell restriction on e capability of a mechanical itch. Above this weight a torte converter is used which imediately gives a 160 tons capality.
The start-up requirement nds to dictate the overall speci:ation which Scammell agrees ith the component suppliers. iton, for example, wants a gra?ability from rest of 1 in 6 and r the company to give full stallation approval insists on lequate engine power.
This restart ability does not )ply as the gross weight goes ). For example, you would not pect a 500 tons gross outfit to start unaided on a 1 in 6 graent. For this sort of operation, ick-up vehicles would be proded.
Scammell designs for a 1 in 25 gradeability at 300 tons and 1 in 10 at 160 tons.
Above this latter weight there is a need for a fully automatic power-shift transmission, but the snag is that automatics do not normally have the required ratio spread. For this reason Scammell puts in an auxiliary gearbox behind the automatic transmission.
The driver must stop to change over the range which means that the vehicle has to be considered as a special and thus cannot operate "conventionally", but it is a useful feature when returning empty as back loads in the heavy haulage are the exception rather than the rule.
Scammell makes its own auxiliary gearbox as there is no other suitable proprietary component available. In the field of "ultra specials" it is often a case of adapting what is commercially available or designing a component from scratch.
The Scammell gradeability parameters are based around a road surface coefficient of friction of 0.6 and a minimum load of three tons on the front axle on a 1 in 10 gradient. As David Picton put it "we have a set of rules which we keep to". This in effect means that on a straight flat road the Scammell ratings can be much higher.
Wynns has used a Scammell at 580 tons on a 1 in 15 gradient for example. It is possible to operate at 1000 tons under these circumstances, ie dead level and dead straight, but this is not an everyday figure to quote as a guide to the vehicle's capability.
Torque converters are extremely useful in this theoretically overloaded situation as they give an operating time margin before the oil gets too hot. Scammell therefore designs with 80 per cent efficiency at the torque converter all day long.
A heavy haulage combination needs a good crawling capability and with a fully automatic transmission in the locked-up mode a speed of 11/2mph is possible. Fully automatic means Allison as far as Scammell is concerned for automotive use.
Most other forms of automatic transmission are not designed specifically for road-going use and are thus out of court on grounds of price and dimensions. As Jim Moore recalled "there is one transmission which is suitable in performance terms, but it won't fit in between our chassis rails".
The basic engine in the Scammell range is the Cummins NTE 350/400. The Contractor 2 uses the Cummins KT 450 which is a turbocharged 18.8-litre (1,150cuin) engine rated at 317kW (425bhp). The maximum torque of this unit is no less than 1,728Nm (1275 lbft).
Above 350 horsepower there is a big jump in the price graph on engines, gearboxes and exles. For example, the Con mender tank transporter uses Rolls-Royce V12 of 21 litr (1,280cuin) capacity which d velops a comfortable 47010 (630bhp). The price is just as in pressive —£15,000.
The Scammell design depai ment often gets salesmen ca ing from other engine cor panies, eg Caterpillar an Mercedes-Benz, but in David Pi ton's words "if we are going f one engine then that engine Cummins."
This is not to say that Scar mell will not consider any oth make; it is a case of if operator comes along with a r uest for something different, 'len he must generate enough usiness for Scammell to upport it.
The axles for a Scammell eavy haulage vehicle are sually of in-house manufacture iith the bogie axles having evel centres and epicyclic hubs. Icammell uses other Leyland its as required.
"We look at what is available nd adapt it to our own product dvantage," said Jim Moore. Vhere an axle has an estabshed on-road rating then it is .erfectly feasible to run it at 30 ler cent overload for heavy aulage at the lower road peeds.
The steering arrangements on Icammells have always had ram ssistance on the other side to -le steering box, thus taking the tresses out of the track rod. This ; now being looked at as not .eing ,particularly cost effective ue to the relatively low frontxle loadings compared with the est of the outfit.
As heavy haulage speeds are nuch lower, Scammell can get ipproval to run at u prated speciications. Although this means hat Scammell has to be very .pacific about the type of operaion to get this approval it is alnost unknown for permission to )e refused. As David Picton put t: "The supplier accepts our Dgic. After all, we've been in the iusiness a long time."
As the road speed comes lown so the load capability goes ip and this is particularly true of yres. A 12.00 x 24 is the biggest yre which can be accommolated and still stay within the '.5m width limit. This can olerate a 50 per cent overload if ised at 1? mph and a five per ent overload at the high speed by heavy haulage standards) of .0mph. Radials with conventional road-going patterns suffice for most applications, with sand tyres being used for higher weights when no suitable road pattern is available.
Cross-ply tyres are not recommended as they can break up at the high slip angles generated during tight manoeuvring.
Suspension is conventional on Scammells with multi-leaf springs being the norm. The leaf spring has to locate the axle and cope with all driving and braking torques and as such is "a very cost effective piece of equipment", to quote Jim Moore.
David Picton backs him up on this: "Why do you want air or rubber? These are usually specified for reasons other than performance characteristics like low maintenance costs or to minimise failures in the bodywork."
In spite of the fact that it is easy to visualise a 200-ton outfit coming to a dead stop very quickly due to gravity, there are, of course, set braking requirements to which Scammell adheres.
The vehicles are not expected to produce a retardation of 0.5g at 300 tons, for example, but there are nevertheless three categories based on weight each with its own braking require
ment. Category A is up to 75 tons and requires a retardation of 0.4g. Moving up the scale to 150 tons at 20mph the requirement is 0.35g while in the 150 tons 20mph Category C 0.3g is sufficient.
The braking system is conventional in that it is full air and Scam operated, but extra compressor capacity is required as the slow speeds encountered are not particularly helpful in keeping up the air pressure.
The cooling capacity of a heavy haulage Scammell has to have a fair margin of safety as it needs to be able to cope with the extra requirements of the power shift and the torque converter. This is why it is so essential to get the overall gearing right.
As Jim Moore explained: "That's the reason for putting in
an auxiliary gearbox on sor specs. We want to use t horsepower and not just as heat source."
The gearing of this auxiliE box ensures that the converter in or near lock up on the steep( design gradients to reduce t coolant load.
As far as materials are cc cerned, the big Scammells a conventional with about the or "exotic" material being the cei mic faces tor the mechani( clutch. Ultra-high tensile stet are not used in the chassis frar because high deflections a needed to get the benefit of thi
gh stress capability. Scammell efers the suspension to delt, not the frame.
There is always the problem of aiding or drilling a high-tensile ime against all recommendapri s. As David Picton exained: "People still weld on to e frame whether we say so or The Craven Tasker trailer shown coupled to a Cadzow heavy haulage Volvo is a typical example of a plant moving/medium to heavy haulage trailer with a payload capability of 110 tons.
This particular design is of modular construction which has a number of advantages, not the least of which are the ability to add extra axles or to substitute a load bed of different length.
In contrast to the Scammell philosophy on suspension, the trailer manufacturers tend to use leaf springs for axle location only. This Tasker device uses hy
draulic suspension with five rams at the rear on each side of the trailer which are inter-connected for compensation.
Although it is theoretically possible to use rubber, Taskers does not do so because this compensation can be a problem even though rubber is satisfactory is a springing medium. The medium really has to be a fluid to be capable of transfer between the axles.
Although the suspension can be raised or lowered to a maximum of 152mm (6in) care is necessary to prevent individual axle overload.
At the front, a similar system is used with two rams on each side. As well as having hydraulic rams, nitrogen accumulators are used to give a high degree of suspension resilience in the unladen state.
All the rams have stainless steel piston rods which Tasker has found to be the only answer to the corrosion problem if the trailer is standing idle. This is a feature of the heavy haulage industry in that as loads get heavier, the utilisation gets less.
Axle weights constitute Tasker's biggest design problem; that and the operator corning along with fixed (and often unworkable) ideas. In the company's opinion, the engineering is the easy bit. The difficult part is finalising a spec that the operator will pay for.
Like Scammell, Tasker tends to use S-cam brakes as the older designs of wedge brakes, although more efficient, were often a maintenance problem and operators have long memories.
This particular trailer has every axle steered. At the re the steering between each e is controlled by track rods a steering links. For normal re use the axles are steered fri the kingpin by hydraulic ra and pipes.
For tight manoeuvring t system may be disconnec and the axles steered by man control with the assistance a donkey engine. • The front.steering wheels go to 90* each way while really tricky situations all • rear wheels can be turned to in addition to the normal p gressive angles.
This feature was difficult achieve in practice. Getting it in was the problem with enot. metal to carry the load.
Designing for heavy haute is a complex business, but at . end of the day it comes down what the operator is preparec pay for.