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Handwheel Design |
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The handwheel translates force input into pressure. The size of the handwheel and the size, thread profile and pitch of the screw determine the ideal mechanical advantage. However, the actual mechanical advantage is considerably less due to friction between internal and the external screw threads and friction at the pressure point between screw and press. Good design maximizes both the mechanical advantage and the efficiency of the handwheel/screw. The Screw All screws are not equal. The profile of a screw thread contributes to its efficiency. Standard screw threads form an angle of 60 degrees where they meet. As the angle formed by the threads increases, the efficiency of the screw decreases. With increasing angles, thread pressure tends to redirect toward the center of the screw rather than parallel to the shaft. For weaker materials, such as wood and low quality steel, wider thread angles tend to compress the thread toward the screw's shaft rather than exert a shearing force parallel to the shaft. For this reason screws made of wood have higher survival rates if the thread angle is approximately 80 degrees or more. The Big Squeeze uses an acme screw made of high quality steel. The threads of an acme screw form an angle of 29 degrees. This lower angle better directs the input force parallel to the screw's shaft toward the pressing job at hand. |
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Where Metal Meets Thread profile combined with the ability to control friction determine the actual efficiency of the screw. As a press screw tightens friction increases between both the male and female threads and between the handwheel and the bearing surface it rides against. The polished surface and proper lubrication of the acme screw in the Big Squeeze limit the friction between the male and female threads. Thrust bearings in the handwheel of the Big Squeeze further increase the efficiency of handwheel/screw combination by substantially decreasing friction between the handwheel and the surface it exerts pressure against. This ensures that more of the input force actually is spent tightening the screw rather than overcoming friction. |
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Mechanical Advantage The large handwheel (about 18" in diameter) combined with the acme screw give the Big Squeeze an ideal mechanical advantage of about 300x. Even assuming a standard 75% loss to friction this leaves a real mechanical advantage of 75x. Every 25 lbs. of pressure applied at the handwheel translates into approximately (25 lbs x 75) a ton of pressure on the platens. Ergonomics The position of the handwheel on the Big Squeeze remains fixed in relation to the press. On many book presses the handwheel moves up and down with the screw, requiring the user to adjust to variations in height as the platen moves up or down. |