FleetWatch Online - 27-GEARBOXES

The gearbox is vitally important when speccing the right vehicle for the job writes Dave Scott, FleetWatch technical correspondent. Read on...

Never mind torque - Nm. South African road transport operators are all 'horsepower-verskrik'. Horsepower dominates transport chat and too often the gearbox is overlooked. Choice of the right truck for an application also involves selecting the right transmission where tractive effort and speed must be evaluated against the background of the type of operation and projected road conditions. Power transmission converts engine Nm into the most usable form to match prevailing conditions. In other words, a gearbox must offer a range of gear ratios. In addition for African conditions, with the diminishing pool of skilled truck drivers, a gearbox must be user-friendly - truck manufacturers take note!

The impact of 'user-friendly' transmissions has been clearly a clincher for Imperial Flexifleet at their Clover contract. Never mind the price, Nissan Diesel UD90 models used in distribution work are being equipped with Allison torque-converter automatic transmissions - purely to overcome downtime experienced from excessive clutch failures and minimise the impact of interchangeable drivers with weak driving skill. An unseen benefit has been a reduction in fuel consumption even with torque-converter gearboxes that are not as efficient as a standard clutched transmission. It's clear, in general, that SA drivers do not know to get the best out of a gearbox.

Far more torque is needed to propel a truck at maximum gross combination mass (GCM) up a gradient than that required to drive an unloaded vehicle along a flat road. And then there's the challenge of start-ability - overcoming total vehicle inertia from a complete standstill. Both situations require massive torque (Nm) multiplication that a diesel engine on its own cannot supply. In other circumstances it is rolling resistance - sand build-up in front of the wheels - to be overcome that off-road conditions create in demanding even more Nm for the job. It's all a task for a gearbox.

The massive increase in engine Nm over the past 10 years has meant that transmission manufacturers have also had to beef up their gearbox designs - matching torque is a critical engineering factor. Just look at the effect of putting 2800Nm through a first gear ratio of 11.78:1 - as on a Volvo FH610 truck tractor - and the output is an astounding 32984Nm! Thirty two thousand NM - eishhhhhh! This is one reason why Cummins are promoting their 'Power-train Protection' engine programmable software. When enabling 'Power-train Protection' features on a Cummins ISX engine, it provides the engine with three different Nm curves and lowers the torque input on take-off thereby protecting gearbox and differential from any abuse that could occur with very high Nm available at low rpm on today's modern diesel engines. Auto-shift transmissions also default to starting in 2nd gear to protect the drive train.

Imperial FlexiFleet’s MD John Loxton – "Driving skills are in short supply. By de-skilling the requirement for precise gearshifts through equipping Nissan Diesel UD90 trucks with Allison torque converter automatic transmissions, we are saving fuel at the rate of 8 litres per 100km in distribution work. In addition we are no longer faced with a clutch replacement every 15000lm."

Opposite ends of the scale
Overdrive and crawler gears are at opposite ends of the gearbox ratio scale. On overdrive transmission is one in which the high (top) gear is less than 1:1. Under good road conditions this enables a truck to maintain a higher road speed with any given engine speed or alternatively a given road speed at a lower rpm. Remember that increasing revs means fuel consumption - lugging in the 'sweet-spot' is what the goal must be when cruising on the open road. That's the serious question to ask - at maximum GCM, where will the driver be on the engine torque curve, in top gear and at the chosen cruise speed for the route?

As an interesting trucking 'side-dish' bit of info, I have learnt that the 'culture of the N1 is 90kph for truckers. The law may state that trucks over 9t GVM are limited to 80kph but the rigs on the route operate at 90kph. If you are in a 'convoy' that has grown en route on the N1 then sticking to 80 means being jostled and left behind which is not acceptable in a driver's attitudinal make-up. Truckers take note that the gearbox and final drive ratios must allow for a 90kph culture - that's the way things are out there.

A crawler gear is exactly what it says. It's meant for crawling out of an emergency. A crawler gear is not meant for taking off from a standing start every time - it's just too slow! So the instruction to a driver that he will use 1st gear for every start can be very limiting and confusing. An example is the 9-speed Eaton FS8209 gearbox fitted to the Fuso 16.253 - the first gear ratio is a typical crawler ratio of 12,64:1. The torque multiplication in 1st gear thus takes the Fuso engine's 708Nm to an output of 8949Nm for a standing start in difficult road conditions on a gradient. The driver must stop the truck when he can start again in 2nd gear.

The forgotten gearbox breather
Gearbox lubricants are very different to engine oils. Gear loading - the result of torque multiplication - means that transmission lubes require a heavier load carrying ability. There's enormous pressure between the teeth.

Patrick Swan, Cape Town-based failure analysis expert, points out that "Gear oil additives are very different to engine oils. Detergency and dispersant characteristics are not essential in gear oil as transmission oils do not come into contact with engine combustion blow-by. Synthetic lubricants work particularly well in gearboxes." The Eaton Auto-shift transmission only requires lube changes every 400000kms when full synthetic oil is used.

Swan is concerned that gearbox breathers are all too often overlooked in vehicle maintenance. "If the breather is blocked," he says, "Then gearbox seals will blow and leak and that's the way that contaminants start to enter a transmission. Damaged breathers are also an entry point for water in high-pressure cleaning. H2O causes corrosion and accelerates lubricant oxidation."

Auto-shift is the future
The Eaton Auto Shift - also known as SmartShift on Freightliner trucks is not an automatic transmission. A clutch is still used only to start and stop the vehicle, but once the truck is under way, it's controlled by shift-by-wire automated shifting, through 18 gears. The driver only has to focus on road craft and not the art of matching road speed to engine torque - all that happens electronically through a SAE J1939 communications data bus where engine and gearbox exchange vast amounts of information every second. And now on the new Freightliner Columbia models the auto-shift mechatronics are into generation III so the technology is maturing rapidly.

All truck manufacturers have auto-shift technology available but at what GVM level and when to introduce this - that is the question. Freightliner's SmartShift is one of the strong reasons behind their success in heavy trucking; they are now the leading stakeholder in the over 440hp truck tractor market segment. The Eaton non-synchro, constant mesh gearbox without auto-shift required serious driver training and when there were failures the default setting always turned up as 'driver abuse'. Even full synchromesh transmissions such as the ZF16S221 were a major problem in the MAN F2000 - overall product reputation can collapse with a gearbox that fails to deliver reliability.

The MAN bad gearbox experience ended with the introduction of the TGA range equipped with MAN TipMatic a robust, 12-speed automatic transmission that talks seamlessly to a electronically-managed MAN diesel engine. A conventional 430mm dry plate clutch, actuated by air under electronic control, minimises slip occurring in torque converter automatic transmissions. As with the Eaton auto-shift design drivers no longer have to operate a clutch pedal and synchronise this with engine revs and optimum torque requirements.

Auto-shift gearboxes are not a complete replacement answer for a fully automatic torque converter transmission. We tend to think of automatic transmissions for tar road operations only, but in severe off-road conditions an Allison torque converter auto-box is the superior way to multiply Nm. Duncan Prince, Product Manager for Western Star Trucks, explains - "If a vehicle has to operate in soft sand or thick mud, pull-away is very tough on clutches, and gearshifts are often difficult because as soon as you break torque to the wheels to effect the gearshift the vehicle stops. This situation is also emulated when pulling heavy loads on steep inclines, a vehicle is 'gear-bound' because it can't shift gear without the load stopping the vehicle dead. An Allison auto on the other hand never interrupts the flow of torque to the wheels while it shifts gear, the advantage of this being that a vehicle that is equipped with an Allison will up-shift under load or incline or soft ground condition where a normal vehicle could not. It will run faster and use less fuel in these conditions because it is running in a higher gear."

Danger - don't look at gearbox issues in isolation
In a forestry application for heavy 4 X 2 trucks, differential failures were first treated through changes in rear axle gear oil specification. On-site inspection revealed that operational condition severity resulted in drivers having to select the first gear on certain gradients. The truck's first gear - a Nissan Diesel UG780 - was not synchronised. This meant drivers had to come to a dead stop from second gear before engaging the first gear. Sometimes the truck would roll backwards and, when the driver dropped the clutch in first gear while rolling backwards, the shock-load would cause differential failure. Changing the crown wheel and pinion ratio from 6,166:1 to 6,833:1 resulted in the truck being able to negotiate all grades in second gear without differential failures.

Never overlook the role of a reverse gear. Efficient transport also needs a truck to work in reverse. For example, concrete mixer units are required to discharge their load as close as possible to the building operations - in most cases this means reversing into a construction site that could be situated on a steep gradient. Reverse gear ability to multiply torque can be a critical component of operational success and that's why detailed route surveys are part of vehicle and driveline selection. It's much more than a route - it's what actually happens at a customer's delivery point.

Conclusion
In the ADE-ASTAS era engines and transmissions were fairly standardised - driver interchange-ability was relatively easy. That's all changed with everyone pursuing their own specification agenda and fleet standardisation has become even more important. The issue that must not be overlooked is that gearboxes are designed to suit a particular function and do not have a universal application. So don't just focus on engine output and vehicle speed. Even traffic conditions are changing the technical profile of truck specifications - do you need an overdrive top gear for average speeds of 30kph? Does the gearbox suit the type of driver? There are many questions that should be asked around an investment in a truck. After all, a truck is a productive machine in which a gearbox plays a major role.

#1 The task of multiplying high engine Nm makes engineering marvels of modern transmissions. A gearbox should outlast the engine if correctly operated and serviced but lack of driver skill is showing in SA fleets where transmission overhaul precedes the first economic engine life.

#2 A gearbox consists of an input shaft, a countershaft below, and a rear output shaft known as the main shaft. Engine torque (Nm), applied at the input shaft, is transmitted to the countershaft by means of a smaller gearwheel on the input shaft and a larger gearwheel on the countershaft. Nm at the countershaft will thus be higher than engine Nm. Several gearwheels of different sizes are mounted on the countershaft - these can be combined with corresponding gearwheels on the output shaft. In this way different increases in Nm supplied by the engine can be selected.

#3 All gearboxes are constant mesh in design - those without synchronizing rings have been misnamed as 'crashboxes' simply because gearshifts are an art without synchro-cones to assist a sliding sleeve engage a gear on the output shaft.

#4 A 'crashbox' is exactly what it means - the absence of a constant mesh design means that ratios are changed by sliding different gears in and out of mesh with each other that creates the noisy 'crashing' effect. This very old technology gave way to the constant mesh designs that are even today used in auto-shift transmissions without synchromesh cones.