Technical Services Notes Sheets

Truck Braking - (getting the best out of truck brake linings)

Truck brakes are mostly of two principal types; cam operated brakes with shoes pivoting on anchor pins (also used on trailers) which are actuated by air pressure, and wedge operated brakes with shoes the tips of which are able to slide on their supports, which may be actuated either by air or fluid pressure. There are also some brakes with sliding shoes, which are operated by hydraulic cylinders between the shoe tips.

Cam brakes may have either one or two anchor pins for the shoes to pivot on and the shoes may have either a hole in the twin webs or a cut-out to locate on or against the anchor pin; the cam is usually of 'S' shape, on the end of a cam shaft passing through a bush in the brake torque plate. The cam usually contacts the shoe through the medium of a roller at the shoe tip and it is important that rollers, cams, camshaft bearings and shoe pivots are all kept well greased and in good condition.

Cam brakes as described are of leading / trailing type and are normally fitted with two tapered linings per shoe, thick at the crown of the shoe and thin at the ends when new; when fully worn, the linings are reduced to approximately constant thickness. Usually, but not always, the two linings on each shoe are a little different and are named the cam end and anchor end'; it is vital that they are fitted in the correct positions and that their thickness at the adjacent ends is very nearly the same.

On the end of the cam shaft is a lever, operated by an air chamber, which usually incorporates either manual or automatic adjustment; levers of this type are called slack adjusters. In a few cases, cam brakes have internal adjustment.

Brakes with sliding type shoes, use linings of constant thickness which should wear evenly. The Girling 2LS two leading shoe brake (which may be actuated either by an air chamber or an external hydraulic cylinder) uses an arrangement of two bell cranks and a push rod per shoe, to cause each shoe to act as a leading shoe for either direction of drum rotation. The expander and adjuster mechanisms, the shoe tips and the cranks must all be greased and working freely; the adjustment on the push rod must just take up any free play. As an exception, the Girling H2LS brake has two double acting wheel cylinders to expand the shoes, which then have no linkage on them; the mechanical parking expander and the adjuster each contact one end of a carrier which has an adjustable central strut which can lift the shoe. Each shoe acts a leading shoe for either direction of drum rotation.

Rockwell Stopmaster, Girling Twinstop and the similar Deutsche Perrot brakes have twin wedge expanders, operated by air chambers one of which is often a spring brake unit; to provide for parking, automatic adjustment is incorporated in the expanders, which are well sealed, so shoe tip greasing is the only obvious point needing attention.

Hydraulic brakes for trucks are not fed from a car type master cylinder but from an air pressure operated actuator; nowadays both the air and hydraulic circuits will be divided, so that protection is provided against a failure in either system. This type of system is known as air over hydraulic (air / hydraulic).

When air is used either to power a hydraulic actuator as above or for direct brake actuation, the system is somewhat complex and has a number of functions to fulfill. The air will initially be filtered and may also be drawn through an alcohol evaporator (to prevent icing in winter) before being compressed; either a governor valve or an unloader may be fitted to let the compressor idle when the system is charged, an alchohol injector or an air drier may be installed and then the air passes into the reservoirs through charging valves.

Either hand or foot control valves take air from the reservoirs and by means of springs and pistons, relate the delivered air pressure to the hand or foot movement; foot valves usually control two circuits for safety reasons while hand valves may be arranged either to increase the delivered pressure or (for spring brake units to decrease it as they are operated).

Brake chambers maybe single chamber, double chamber or of spring brake type; double chambers permit a brake to be operated by either of two separate systems, while spring brake units allow a powerful spring to apply the brake for parking when air is released from the associated chamber. To speed brake application the delivery from a control valve is often used as an input signal, to a relay valve near to the brakes which is fed directly from the reservoir; there may also be quick release valves near the brakes, which open as soon as the line pressure falls and discharges the air from the brake chambers.

Trailers carry their own reservoir and are connected to the tractor unit by two or three air lines; the names for these vary but the most meaningful are 'Supply' for the one that supplies air to the reservoir (may be coloured red), 'Control' for the one that supplies the service braking control signal (may be coloured yellow) and, if a third line is used, 'Secondary' for the one that provides for secondary braking (may be coloured blue).

The operation of the trailer brakes is either (1) via a relay valve, in response to a signal from the tractor through the control line for normal service braking (2) in three line systems, via the secondary line direct to the secondary chambers of dual air chambers or if the 'Feed? line should be severed, as in a break away situation, the relay valve incorporates an emergency function which applies the trailer brakes immediately, using the air in the reservoir.

Truck braking may be modulated by load sensing and / or anti locking equipment, but this is dealt with separately. (getting the best out of truck brake linings)

Contact us now for guidance and advice, or to discuss a specific project, by telephone on 01254 397561, by fax on 01254 389722 or by email at david@frictioncomponents.co.uk

"Solving Industries Braking Point"

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Friction Components and Systems Ltd - "Solving Industries Braking Point"
Unit 103, 25 Clydesmill Road, Clydesmill Industrial Estate, Cambuslang, Glasgow, G32 8RE

part of the Friction Components and Systems group of companies