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Know Your Air Brakes

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27th May 1966

Page 92

Page 92, 27th May 1966 — Know Your Air Brakes

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Part 10 The Double Check Valve

ON the modern, articulated outfit, where it is necessary to control braking by separate valves—that is, foot pedal or hand lever—there has to be a means of isolating one operating line from the other. This is achieved very simply by the double check valve, which is a three-piece body with bracket for chassis mounting.

The valve has a straight-through passage in the form of a bush, with a bobbin or shuttle acting as the free valve, the end caps of the valve body each retaining a seal, and the free bobbin can register on one or the other and seal off either end of the valve body. There is a centrally arranged port to deliver air from either control to the wheel chambers.

Each end of the valve accepts a line direct from a control valve, and the action of the double check valve is simple and reliable. On one or the other of the operating valves being opened, air will flow into the double check valve and push the free bobbin to the other end of the valve body, sealing off that end of the valve. The length of the bobbin is such that it is clear of the outlet port when at either end; therefore, the air that has driven the bobbin to the opposite end of the valve will hold the bobbin on its seat and at the same time feed the brake chambers.

If the first operating valve is closed and the second opened, the bobbin will change ends and air will flow again to the wheel chambers. In the event of both operating valves being opened at the same time, the higher pressure will move the bobbin and only one control will apply. This valve should always be mounted in the horizontal position and should have an occasional nipup to ensure freedom from leakage at the seals.

To test the valve for leakage in the lines it is necessary to apply soap suds to the exhaust ports of the control valves; then, if on operating a valve, the leakage exceeds one bubble per second, the seals in the double check valve should be renewed. If the double check valve is unable to seal a Line fully—for instance, when the foot control valve is opened—the leakage will show at the exhaust port of the hand-control valve and vice versa.

Similarly, to test the action of the valve, apply the foot control and note that all brakes are operating fully. As the footbrake is released, so should all brakes release smartly; on application of the hand-control valve, the trailer brakes should operate fully and again release cleanly. This indicates that the double check valve is in good order.

This last test does, of course, apply to pre-plated units and trailers; on plated equipment, which I will cover later, the hand control operates the separate safety system and is now arranged to operate the tractor driving axle brakes as well as the trailer brakes.