April 2007

	Father and son team has 45 years experience between them. Ian Banyard has seen 32 years of chassis extension and propshaft matching while Richard Banyard joined Pretoria Engineers 13 years ago to establish a formidable source of intellectual capital in the chassis extension business.

In this, the fourth article in our series on drivelines, FleetWatch technical correspondent Dave Scott focuses on propshafts.


Known elsewhere as a drive shaft, the word 'propshaft' is typical South African trucking terminology. As it's under the chassis and is 'just a tube', it's a bit transparent to most people and for many technicians as well. But ignoring a propshaft is perilous because it will fail on the road at a point of maximum stress. If it's a failure on a steep decline, a truck can run away as there's no engine compression to assist retardation. And if it's a short propshaft that drives a concrete mixer unit, the setting cement inside a mixer heightens the sense of crisis.

An unplanned propshaft failure means no choice - it's either extended downtime while a new propshaft is made to spec or repaired or it can mean limited downtime while you buy an off-the-shelf, original equipment (OE) unit at gold-plated prices. OE propshafts are known to be outrageously expensive.

A truck propshaft picks up multiplied gearbox output torque - anywhere up to 32 000Nm in first gear - and hurls this twisting force at an axle. A propshaft lives in a dynamic condition where it must take up the vertical movement of a drive axle at the rear with flexing gearbox mountings at the front. On a corrugated dirt road, a spinning propshaft would 'chatter' if all these movements and thrusts are taken into account. High-speed, return-empty trips on poor road surfaces will accelerate driveline wear and shorten propshaft life. This means a propshaft has to be tough enough and is a safety critical item.

Safety critical
Slowing down is just as important as accelerating and cruising. A propshaft is a main component of road transport retardation. The majority of today's extra-heavy rigs rely on powerful engine brake systems to provide reverse thrust forces into rear axles to save foundation brakes and increase safety. A propshaft carries this energy.

A propshaft should not be viewed in isolation - it's an integrated driveline component. High clutch wear will also reflect itself in the propshaft. A driver who revs and drops a clutch to get moving is causing propshaft failure. 

Most often the costs of propshaft failure extend well beyond the component itself. A loose propshaft thrashing around until the truck comes to rest often results in more consequential damage to other chassis-mounted parts.
 

Centre-bearings play a vital role in a drive train. Propshafts cannot be too long as this causes ‘whip’ and torsional vibration. A centre bearing allows correct propshaft lengths to be installed and must be at right angles to the propshaft to operate effectively.

Grease nipples – especially a nipple situated inside a universal joint – are easily overlooked in routine service. If it’s got a nipple it’s just ‘got-to-be-greased’ and not overgreased. Make sure it’s minimum standard EP2 grease as well.

Grooved propshaft-flanges spread and absorb the stress of twisting Nm effort into the propshaft with fewer bolts required – in this example of a heavy-duty propshaft, only 4 bolts are needed.

Grooved propshaft-flanges show how the two grooved flanges ‘bite’ each other to ensure there’s no movement in the connection when maximum torque is applied.

'Maintenance-free' is a propshaft misnomer
All 'maintenance-free' means is that greasing is not required. The term can be misinterpreted to imply that a driveline does not need attention. This attitude is fatal. Propshaft splines and universal joints may not require greasing but operational conditions may accelerate wear to the point that serious torsional vibrations start to destroy the gearbox and axles. Sealed units must always be inspected for bearing-play.

Where a propshaft flange is situated too close to an exhaust system, heat radiation spill-over will shorten propshaft grease life. This shortens service intervals or accelerates universal coupling failure.

EP2 grease is recommended for use on propshafts. Failure analysis expert, Patrick Swan, observes that driveline greases should contain molybdenum disulphide to prevent metal-to-metal contact. 

The correct fitting propshaft bolts and nuts must be reused. If a propshaft is disconnected for repair or vehicle towing - propshaft OE bolts and nuts come around an eye-watering R40 per bolt and nut. Nylock nuts may not be re-used.
 

Propshaft splines are where operating conditions can cause accelerated wear – even if they are Teflon-coated.

Propshaft angle - product of careful calculation
Pretoria Engineers - founded in 1931 - have seen more propshafts go through their engineering shop in Hermanstad, Pretoria than they care to remember. The company has detailed records going back over 10 years for every propshaft that they have modified and fitted.

Managing director, Ian Banyard, explains why they do this. "Trucks last at least 10 years and then we get calls from either the first or second owner when it's time to rebuild a propshaft. We must be able to provide accurate info."

Richard Banyard, Pretoria Engineers' financial manager, plays an active role as well in vehicle homologation and database management. He also maintains the spreadsheet formulae that drive propshaft angle and length calculations. He makes the point that it's a formulae-driven business.

"We work to a safety factor of 65% and while formulae provide us with exact limits, it's dangerous in our operating environment to take any vehicle engineering to the limit."

Pretoria Engineers recommends a minimum propshaft angle of 1 degree for efficient operation with a maximum of around 8 degrees. Richard states, however, that "it's a 3-dimensional issue and 8 degrees is not the absolute limit - it could be a 10 degree angle and entirely dependent on formula outputs."
 

A propshaft engages the 3rd differential in the first axle of a twin-drive rear axle set. The 3rd diff is there to take out any fight between the two drive axles that will travel at different speeds. Without the third diff propshaft wind-up is inevitable. But a badly balanced propshaft can transmit torsional vibrations that can destroy both the 3rd diff and the interwheel diff on the first drive axle.

A violent driveline enemy - torsion vibrations
A common problem with driver rotation is that drivers fail to report driveline vibrations. As far as they are concerned, driveline vibration is just part of a truck's characteristics, whereas a driver dedicated to a truck will notice this problem and report it before it achieves major failure status. This requires a technician to take a vehicle for an on-road test. How often do vehicles get road-tested in beleaguered service operations? 
 

Propshaft universal couplings are Nm task-matched. Universal joints would not be required if a propshaft was designed to run perfectly straight. A drive shaft can never remain straight because a rear axle travels up and down on its suspension, exaggerated by road conditions.

Conclusion
The propshaft business is a big one. Just pick up a phone book to see how many propshaft experts there are! And of course, it's subject to varying quality standards that aren't policed. So there's the usual flood of sub-grade parts.

It appears that the SABS just cannot control what is happening to chassis modifications out there. Truck dealers and transport operators are taking short cuts on homologation and chassis modification partly due to ignorance and mainly due to price. Some wise observer made the comment: "If the price is too good to be true, then it's too good to be true!'

A propshaft is expected to last the life of a truck but don't bank on this because it's a highly stressed component twisting Nm into a differential.