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IN YOUR OPINION
Page 97
Page 98
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Handbrake Efficiency nURING the past few weeks, some of your correspond ents have expressed views regarding methods of achieving a reasonable handbrake efficiency on commercial vehicles. In the particular case of the heavier commercials which are normally fitted with single-leading, single-trailingshoe cam brakes, a high input per brake is required to obtain a reasonable handbrake performance. From the operator's point of view it is essential that vehicles such as this, which are in the higher price bracket, have a reasonable life between docking periods. Consequently, the period between brake adjustments should be made as long as possible, having due regard to the disposition of the various levers in the handbrake linkage, and ensuring that these do not move too far from their optimum positions in relation to.the rods and so on connecting these levers, so that loss of effort through angularity is not appreciable.
There is a limited amount of travel available in the cab for handbrake lever movement and this has to take into account brake-shoe running clearances, drum expansion and an allowance for liner wear. Consequently, there is a maximum fixed lever ratio which can be adopted and often this is found to be insufficient to provide the brake input efforts required at a reasonable hand effort. The difference, however, can easily be provided by use of airpressure power assistance. This company, having seen the need for more powerful handbrake systems giving adequate shoe movement, has developed a system of handbrake power assistance which can be adapted easily by any vehicle manufacturer to suit his own particular vehicle chassis layout. By providing the handbrake air circuit, with its own protected storage capacity of compressed air, a fault occurring in either the service brake or the handbrake power systems cannot affect the behaviour of the remaining system. Consequently, the rear brakes can be applied by power, even in the event of a failure of one circuit, and locked on mechanically by an orthodox, simple, single-pull handbrake linkage.
The components used to provide power assistance of the handbrake linkage are no more complex than their equivalent counterparts in the orthodox service brake system and require no more complicated maintenance than the compressed air brakes on thousands of vehicles already fitted with this type of equipment. In fact, the similarity between the various components is so marked that no new techniques have to be learned regarding their servicing.
Two other points are worthy of mention. First in the arrangement recommended for operating the. handbrake, air pressure is used only to apply or release the brakes, the power system being exhausted of air once the handbrake has been applied and the ratchet engaged. This has two benefits—namely, that the driver is aware, before he leaves the cab, whether he has sufficient holding power for the conditions under which he has parked and moreover, it is impossible to release the handbrake unless sufficient air pressure is available in the whole air-brake system to give satisfactory braking for normal road work.
Secondly, by choosing servos for hand and service brake actuation which, whilst independent as regards the airpressure circuits, are nevertheless interconnected, it is possible to provide a power input to the rear brakes of identical value, whether the service brake or the handbrake is operated. The only difference is that should the handbrake be operated, the driver's manual effort will be added to the power load into the brakes. From this it will be seen that no matter whether the driver operates the service brake and the handbrake together, it is impossible for him to overload the brake assemblies, possibly causing high temperature conditions and excessive liner wear. In view of present-day tendencies towards increased speeds it is essential that adequate braking power should be available to meet all contingencies, even the relatively remote possibility of a fault occurring in part of the air-pressure system. The fact that the rear brakes, at least, can be operated either by the service brake or the handbrake with full power input, is therefore of great importance, particularly when it is realized that a high degree of overload on these brakes is also impossible.
Lincoln. D.H. BALLARD.
Technical Director, Clayton Dewandre Co. Ltd.
" integration "—and Labour I AM surprised at the doubts expressed by some of your correspondents concerning the Labour Party's transport policy. Party spokesmen, when they have been coaxed into discussing transport, have repeatedly used the word "integration ". As the verb integrate means form into a whole, there should be no doubts. The Labour Party obviously intends to nationalize transport, unless they have conjured up some new meaning for integration.
Integration is not the answer to our transport problems, as it lends itself neither to efficiency nor to customer satisfaction. There is only one way to make transport highly efficient, and that is by co-ordination. If the last term of nationalization is any indication of what can be expected from a Labour Government, may I say that independent operators, some of whom are already operating co-ordinated services, are better able to do the job properly.
"Kilmacolm, Ren f rewshi re. JAMES M. WATSON.
Support for the Trolleybus I FIND it reassuring to read in your columns (Letters of January 3 and 17) at least some support for the inoffensive trolleybus.
I have been deprived of my local trolleybus service in Newcastle just recently. I can vouch for your correspondent's remarks about the consumer's protests going unheeded. I have protested at having Hobson's Choice between an Atlantean or Shanks' Pony. The official reply is that motor buses have been approved by the City Council and the Traffic Commissioners. The trolleybuses are found to cost more to operate than motor buses, but, when
a switch takes place, the replacement service is less generous. If a comparison is to be valid, then a• like-for-like service basis should be taken, and not with fewer vehicles or stops. Incidentally, if you should lose your local bus stop during conversion, you can console youself that the exercise will do you good. since the move has the blessing of the authorities.
Perhaps the best I can offer is to reiterate what your correspondent Mr. Woods wrote and urge that the trolleybus be developed into a suitable vehicle for the Buchananage city. The Report (chapter 1 paragraphs 24-31) holds some rather unpleasant truths for the p.s.v. operator if one accepts Professor Buchanan's criteria for a good urban environment. Electricity could resolve the difficulty.
Newcastle upon Tyne, 3. JOHN PETRIE.
Exit Oilbath?
AM surprised at Handyman's bland statement in your I issue of March 6 that the oilbath is "undoubtedly the most successful type of air cleaner' and can assure him that this is not so. For instance, its efficiency varies between 95 and 99 per cent, according to type, engine speed, condition, etc., whereas a good paper filter is always 99.8 to 99.9 per cent efficient.
He illustrates his point by referring to a paper filter that became clogged and stopped the engine. Obviously this was either too Small for the job or had been wantonly neglected, or both.
If an oilbath is similarly neglected and fails to stop the engine, then a large portion of the accumulated dirt will pass into the engine in large dollops until it clears itself, and subsequently all the dust and dirt in the atmosphere will follow suit until it is serviced. If the filter is saturated where else can it go? What is best for the engine—to stop it, or let it run on towards destruction?
Changes are on the way, and there are now on the market restriction indicators that show when an air cleaner needs servicing, paper air cleaners preceded by efficient multiple cyclone units which prolong its life by at least five or six times, and for very severe conditions (such as quarries) a self-cleaning unit which will maintain an efficiency exceeding 99-9 per cent indefinitely. So the oilbath is on its way out.
Gilwern, nr. Abergavenny, Mon. C. M. STREETE.