Brake points to watch in service
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HE BRAKING SYSTEM is one )f the more important areas of a :ommercial vehicle. That much ;hould go without saying. So it ollows from this that great care ;hould be taken in setting up the >rakes and in their subsequent naintenance.
Regrettably, this is not ilways the case. As one of the najor manufacturers in the iraking industry, Girling has a lreat deal of experience in what an go wrong with brakes and thy. I have been talking to Stan lulme, service promotions lanager at Girling, about the lore common faults encountred by the service engineers.
Perhaps the most telling of :tan's comments was: -I am no )nger surprised by some of the tupid things that people do eith brakes."
With hydraulic and air/ ydraulic systems, the major wit — by far — is contaminaon of the seal rubber. This is used in the first place by conimination of the fluid in some
In Girling's view, the use of icorrect fluid from the otorious "spurious manufacirers" can cause a lot of prolems to operators. Hydraulic uid has to withstand extremes F heat (because of its proximity 1 the friction surfaces) and exemes of cold (if the vehicle is 3rked outside overnight). It so has to act as a lubricant for le rubber and to be chemically ert.
This latter point is vital as it is _lite possible for the fluid to be contact at one point or anher with rubber, aluminium, mild steel, nylon, plastic, brass and copper.
With the spurious fluid, this is produced to a very basic specification which in Girling's opinion leaves a lot to be desired. For example, the minimum fluid specification calls for a three-day immersion test of the rubber seals. A maximum swelling of around three per cent is permitted but the specification does not lay down what must happen after, say, one week or one month.
In contrast to this "threedaytest, Girling and the other recognised manufacturers test for 1 8 months to check on seal swell.
For all commercial vehicles Girling recommends the crimson brake fluid. This has a slightly lower boiling point than the so-called -universal" fluid — 210'C against 2E38cC (410c•F against 550'F) but in spite of this Girling recommends the crimson type because of the different requirements of a brake fluid for commercial
vehicle and private car usage.
With cars, spot heat can cause trouble around the calipers on a disc brake system, so the main requirement in this case is resistance to heat. With commercial vehicles and drum brakes, the drums themselves have a greater heat capacity. But because of this relative increase in size of the components, the major problem area here is the large area of seal rubber and its lubrication requirement.
Thus the universal fluid is by definition a compromise for all types of operation whereas the crimson fluid has greater lubricity, essential for commercial vehicle use.
Girling stresses over and over that many chemicals can cause seal deterioration, eg petrol, paraffin, derv, carbon tetrachloride, or lube oil. Depending on how much is pre sent, the contaminant can cause seal swelling in one day or one month. If the driver is lucky this will cause brake binding so he will stop to investigate. If he is unlucky he will get total brake failure.
According to Girling, contamination of brake fluid can always be avoided. And this includes not handling the rubber seals with oily hands.
Adjustment
Another big headache for the operator and the brake manufacturer is lack of regular brake adjustment. This introduces overstroking of expander components, excessive movement of the diaphragm and seals and so on. The requirements for brake adjustment obviously vary with the type of operation — a trunker might not have to adjust as often as a tipper for example. As an approximate guideline, Girling recommends a once-a-week check on brake adjustment.
In the future it is quite likely that some form of automatic adjustment will become cornpulsory.
As far as Girling is concerned there will be a great need for an education programme to get the operator and the workshop staff to move into a new area of technology possibly dictated by legislation. Advice on automatic adjustment is freely available to anyone who doesn't understand how such systems operate. The Girling technical service department, for example, can be contacted on 021-553 2969.
With many commercial vehicle applications it is impossible to carry out a lining inspection without first removing the drum and hub assembly. This is something of a sore point with the brake manufacturers who usually provide a hole in the backplate for inspection purposes. However, they cannot
control how the chess manufacturer installs the equii ment. The worst problem is tF large multi-leaf spring assen blies on the rear axle which g. in the way very effectively ii deed. The front springs do ni pose quite the same difficulty . access.
Many operators go for th brake lining which they thir will give them the longest life the cheapest price. Understan, able — at least in theory.
Girling does not in fa, recommend relining of woi brake shoes, but admits that it easy to pontificate from tb company's angle. There is reason for this, however, and hinges around the ways ar means of doing a reline.
The list of things that can wrong with a reline is long. F, example, rivets can be USE which are too long. The wror lining material can be use. Riveting in the wrong sequen( can set up undesirable stress. in the shoe. If the rivets a pulled up too tight, this w cause the lining to split.
One important point remember is that many brat shoes are "standard" cor ponents which have to accor rnodate different types of lmint dependent upon the chassi This means that there will be number of rivet holes on tt shoe not all of which will used at any one time. Thus if tl relining is not done by an e perienced company it is pe fectly feasible to reline the sho using the wrong holes.
Another common proble with brakes is lack of grease the adjusters and expand units. These should be pack( out with grease during SE vicing.
Any sliding metal-to-met joint needs grease, but for bra systems it has got to be of ti high-melting-point type. TI iirling brake grease for example s a mineral (lithium) based irease and is intended for the iraking system mechanical, ornponents only.
If it is necessary to take the rake drums off during routine iaintenance and spring brakes re fitted then Girling recomlends that the air pressure be ept up to hold the springs in ompression while the wind-off olt is released.
Obviously there will be mergency cases when no air is vailable. In these cases it is still ossible to wind back the rakes, but this means winding ie bolt thread back against the ill spring loading — and its a trong spring and a very fine read .
Girling has found that incor)ct shimming of the master ilinder to the air chamber can ause trouble as far away on the ahicle as the compressor. Nortally there should be a gap of .8mm (1 / 32in) between the nuator push rod and the piston I the master cylinder. If this ap is decreased, then the inifficient clearance causes the rakes to bind.
At the other extreme, with lo large a clearance, the diaphigm has to work much harder,.
move further. The vehicle can lus run into an air capacity -oblem because of the excess e movement leading in the ng run to a highly overworked )mpressor.
This latter problem can also ) caused by a selection of rni)r air leaks. Girling has often und that a fitter will spend )urs looking at the air brakes to id a fault when it is simply matter of not enough air getig through. In other words it is isential to get the right press-e at the air chambers. Overheating of brakes can be iused by a number of different ctors, but there is one which iuses Girling concern because IS out of their hands. This is here the operator specifies a g box body on a chassis with 5Tinch wheels and 13-inch .akes. With this particular ,mbination it is very difficult deed to get enough cooling air to the brakes — aggravated by e type of stop/start operation ually undertaken by this class vehicle.
As I mentioned this is outside a influence of both the brake
chassis manufacturers. As are is no way that air can be icted in efficiently, it is a proDm which Girling has to live th, and it recommends that th these vehicles on stop / )rt operation, the brakes are ecked more often.