AT THE HEART OF THE ROAD TRANSPORT INDUSTRY.

Call our Sales Team on 0208 912 2120

PROS AND CONS OF THE TRANSMISSION BRAKE.

12th February 1924
Page 20
Page 21
Page 20, 12th February 1924 — PROS AND CONS OF THE TRANSMISSION BRAKE.
Close
Noticed an error?
If you've noticed an error in this article please click here to report it so we can fix it.

Which of the following most accurately describes the problem?

Some Facts and 'Figures Relating to a Widely Used Method of Braking.

IT IS A legal and practical requirement that two independent brakes should be fitted to all motor vehicles. One of these always; operates upon drums secured to the rear wheels, but there are a number of possible positions for the second. In the majority of British-built lorries the second brake operates on the transmission, the favourite position being' behind the gearbox.

It is rather curious to notice that, with the exceptionof a few well-known lorries, such as the PierceArrow, the transmission brake is not 'usually found en American vehicles. This fact has sometimes been used as a support for arguments against this type of brake, but the writer believes that the real reason is that, generally speaking, the duty of the brakes on lorries operated in America is not nearly so onerous as that of brakes used in this country. This follows from the fact that the inter-urban heavy vehicle traffic in the U.S. is very small. To commence with the advantages of the transmission brake ; first and foremost there is the multication of braking effeet provided by the final drive reduction. Thus, if the worm or bevel drive gives a gear reduction of, say, 6 to 1, the torque applied by the brake to the propeller shaft is multiplied by five in its transmission to the rear wheels.

This means that, even though the diameter of the transmission brake drum be somewhat less than that of the drum on the rear wheel, the brake will still be far more effective than a rear-wheel type, and its operation entails a smaller effort on the part of the driver.

Another important advantage is that the braking effect is transmitted through the differential and is, therefore, correctly distributed between the two rear wheels. Of course, in most cases an equalizer bar or whipple tree is incorporated in_ rear-wheelbraking systems, but these devices can only equalize the pull on the rear-wheel shoes ; they cannot dis tribute the actual braking effect. . Turning to the disadvantages ; the principal criticism_ levelled against the transmission brake is that it imposes additional stresses on the propeller shaft, universals, and final drive. This is not a really serious matter, because these parts must in any event be made strong enough to withstand the heaviest engine torque (on low gear) and this torque is about as great as the maximum imposed by the brake

There is, however, the fact that when the clutch is disengaged and the brake suddenly applied, the torque on the driving parts is rapidly reversed, and this may lead to fatigue fractures and trouble with the universals. This trouble can be obviated by slightly increasing the size of the parts, but it is interesting to record that the Scotland Yard authorities do not look upon the transmission brake with favour for Metropolitan buses, because failure of the transmission will put the brake out of action.

Alternative Positions for the Transmission Brake.

The stresses on thp propeller shaft and joints can be avoided by placing the brake on the tail end of the worm-shaft (as on the Halley) or directly in front of the final drive (as on the Burford). However, in these positions the brake adds to the unsprung weight, and the operating gear is necessarily more complex than when the brake is placed behind the gearbox. In the latter position the brake does not move relative to the chassis, so that brake snatch, which is caused by. violent axle movements, cannot occur.

Another criticism, which must be taken into account because it is so often brought forward, is that the torque reaction is suddenly reversed when the brake is applied, and that this may produce harmful effects on the vehicle. While this argument may carry some weight in the case of light cars

travelling at high speeds, the following figures will suffice to show that the reaction effect is neglible in the case Of a van or lorry. First of all, it must be made clear that the reaction comes into play owing to the tendency or the shoes to be dragged round by the drum. As the shoes are anchored to the chassis, this tendency causes a transverse torque or turning effect to be applied to

the vehicle' and this torque will always be' precisely equal to the braking effect on the drum. Now, the maximum braking effect which can be imposed is that which will nearly, but not quite, lock the wheels, and this can readily be calculated. To make matters clear, we will take a 30-cwt.. van as an example, with a back axle weight of 4,000 lb.2,000 lb. on vach rear wheel. When the wheels are about to slip the force applied between tyre and road is roughly half the downward load, that is, 1,000 lb. in this case.

With wheels of 30ins. diameter, the radius at which this force acts is 11 ft., so that the maximum torque which can be produced on each wheel by the brakes is 1,250 lb.-ft. The total for the two wheels is, therefore, 2,500 lb.-ft. Then, if the final drive reduction be 5 to 1, the actual torque on the trans mission brake is 2,500 5 =-• 500 lb.-ft. This will,

therefore, be the magnitude of the braking torque reaction.

When t•he engine is driving the van, at full throttle and on bottom gear, there will be a tractive torque reaction which is usually about equal to the Maximum braking treaction, so; that, in these circumstances, suddenly declutching and applying the brakes will cause a reversal of torque on the chassis from + 500 to — 500 lb.-in.

To get some idea of what these values mean, suppose that the interior of the van is wide enough to give a heavy iron roller, weighing 200 lb., room enough to Move 5 ft. from side to side: Then, viewed from the back, when 'the roller is on the right as shown in the sketch, its effect is equal to that of the

maximum tractive torque-200 2i = 500 lb.-ft. Rolling the weight across to the dotted Position on the other side is equivalent to the effect. of declutching and applying the transmission brake with full force.

'It will, therefore; be seen that, even under the worst conditions, the reversal of torque on the chassis is 'not sufficient to trouble about, unless combined with very rapid cornering.

Tags

Organisations: Scotland Yard

comments powered by Disqus