DESIGNED FOR EASIER MAINTENANCE

By F. M. Lincoln, AM.I.Mech.E., Chief Development and Experimental Engineer for commercial vehicles The British Motor Corporation Ltd. Birmingham.

Reproduced from "Machinery Lloyd and Electrical Engineering" (Vol. 38, No. 16 - 27th August 1966) with kind permission of the author and the publishers.

Commercial vehicle maintenance is becoming an increasing problem due to the rising cost of materials and labour, the difficulty in obtaining qualified mechanics and suitable supervision, and the increasing pressure to make the vehicles available for more hours in the day. It is natural, therefore, that considerable time, thought and money have been put to the reduction of the need for servicing.

Many features have been introduced over the last few years to this end. Tilt cabs to improve access to engines with forward control vehicles are a typical example. Or take no-loss cooling systems to provide a reduction of time in checking water levels; occasional checks on the water level are desirable but take only a few seconds. Then, allied to this, is the use of improved antifreeze solutions which can be left unchanged for two years, thus saving bi-annual draining and refilling.

While on the subject of the daily check, one could mention the additional feature of an oil level indicator to avoid the old routine of checking the dipstick. The dipstick is still fitted as an insurance against possible malfunction of the electric oil level indicator.

Going up the scale in terms of periods between checks, there are improved battery designs with a manifold or a revised top cover in place of the old separate screwed plugs for topping up, where time is clearly saved. The provision of battery condition indicators in the cab will also prove advantageous.

Improved lubricating oils are now becoming available which permit increased oil drain periods. Double the original period, i.e. 6 000 miles instead of 3 000 miles, is frequently claimed by the oil manufacturers and with considerable justification in most cases. Gearbox and rear axle change periods can certainly be increased and in some cases, a yearly change would be satisfactory. Claims for car units have been made on a filled-for-life basis, but this is perhaps going too far for commercial vehicle applications.

Filtration for engine, fuel and engine air supply is still a problem in so far as no radical functional improvements have been made over many years. The only solution to reduced frequency of filter change has been to provide larger filtering elements or more sophisticated means of filtration, i.e. the addition of a centrifugal filtration stage to take out the larger particles. Even here size is perhaps the main criterion since even the centrifugal cleaner needs cleaning periodically.

More efficient air filtration by two-stage filters also has the indirect effect of reducing servicing by reason of the reduced bore wear and hence increased life of the power unit.

A further improvement has been the introduction of a water separator in the suction feed line of the diesel fuel system. This will prevent sludging and corrosion of fuel pipes and pumps, thus reducing the possibility of pump failures and possibly increasing life between overhauls.

Turning now to perhaps the most difficult problem. i.e. the elimination of lubricating points, one is faced with probably the biggest sphere for improvement and one which has not yet been fully explored. Automatic chassis lubrication (ACL) springs to mind immediately as cutting out the major problem-that of labour. However, ACL is expensive as installed, wasteful of oil and is inclined to leave its trademark wherever a vehicle has stood.

It is in this sphere that improved materials, now coming on the market, may provide the answer. Polymers resin bonded fabrics, impregnated graphite, etc., provide convenient bearing materials that are not affected by water or corrosion and have a low coefficient of friction. Unfortunately, they do not have the load-carrying capacity of the conventional bearing materials and are usually susceptible to damage by dirt and corrosion of the usual steel journals employed with them. Chrome plating of the journals is helpful but expensive. SeaIed assemblies can be the answer here, but when thinking of components. such as shackle pins, steering knuckles, etc., their high loads, both radial and axial, complicate the sealing problem. It is perhaps in the field of smaller components, such as handbrake linkages, gear change mechanisms, clutch, and brake pedal pivots that synthetic materials will find a most immediate application. These are applications where axial loadings are lower and water and dirt less frequent to make sealing critical. Developments are also proceeding with the combination of load carrying materials, e.g. phosphor bronze with surface impregnation by the synthetic materials mentioned.

A field where synthetic bearing materials have already taken a hold is that of track rod and drag link assemblies, wherein the nature of the construction the joints can be sealed in the beginning without complicating the assembly to the vehicle, in contrast to the handbrake systems which are complicated linkages assembled piece by piece to the chassis. There is no reason, however, why the design of such components may not be modified to incorporate sealed bushings and assemblies.

The inferior load-carrying capacity of most synthetic bushings will necessarily increase the size of the component. This is unlikely to be an acceptable answer in all cases.

In contrast, there are some materials-molybdenum disulphide is perhaps the best-known example that reduces friction. Here a very considerable advance has been made, limited only by the ability to keep the material permanently on the surfaces. Despite claims, the "lubricant" has a habit of disappearing. Means of attaching the molecules of these low "mu" materials to steel are no doubt being explored but so far no real commercial breakthrough has been made. This is as applied to simple journal bearing applications where dirt, moisture, and loading tend to remove the lubricant. Under well-sealed conditions no doubt the statement could be disputed.

As regards brake systems, automatic shoe adjustment is now available and this eliminates the need for continual checking. Transparent brake fluid reservoirs are also being-introduced in positions where they can clearly be seen, thus reducing the time needed to check fluid levels.

Mention must also be made of corrosion of cab and chassis. This may be a rather long term servicing problem, but it is still servicing and, with the increasing use of salt during winter months, is fast becoming short term. The use of galvanised materials is the more common answer today, but to a plate, everything is very expensive. Spraying of assembled chassis frames inside is a possibility, particularly to protect brake pipes that are vulnerable to the packing of corrosive materials inside frame members.