By J. Carvill
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The flow results in a pressure difference across the piston and the main valve opens to allow the oil to flow to a tank. Non-return valve, check valve These are valves which permit flow in one direction only. 150 Hydraulic relief valve 35 In one type a ball, needle or disk, etc. is held onto a seat by means of a loaded spring and it opens with the fluid flow. In another valve a spring-loaded piston covers a port, and a third has a hinged flap held closed by gravity and opened by the flow of fluid.
This is a hinged joint, connecting two or more rods, used as a tie for roof trusses. 181 Pin joint for ^WC- — r -I 1 rods 41 C O H E R E D JOINT A rigid joint connecting two rods in which a cotter pin is used. 8 BEARINGS Bearings are used in engineering to restrain or guide the movement of one machine part relative to another with the minimum of friction and wear. In most cases they are used between rotating parts, such as shafts, and machine frames, but they may also be used for linear motion.
The hub is often made a sliding fit on the shaft where axial movement is required. 174 Square A keyway is a slot in a shaft into which a key is fitted, and it is usually produced by milling. Splines T Y P E S O F KEY K E Y S A N D K E Y W A Y S IN S H A F T S A key is used to prevent a machine part from moving relative to another part in a given direction. On shafts they prevent the rotation of a part relative to the shaft but may allow sliding along it. The key must be strong enough to transmit the shaft torque.