X-Y-Z: Putting the Clamps on Motion
Clamps are used in fixtures for processes such as welding, adhesive bonding, pressing, riveting and leak testing. In material handling applications, clamps are used to grip parts in overhead transfer systems or to stop parts traveling on conveyors. Clamps can even be used like presses to install small parts, such as plugs.
Clamps can be powered manually, pneumatically or hydraulically, says Dave Doberstein, senior designer at Aladco (Waukesha, WI). Hydraulic clamps produce greater force than pneumatic ones. They are also more stable and move more smoothly. On the other hand, pneumatic clamps are faster, cleaner and more economical. Either way, a check valve should be installed with the cylinder to maintain clamping force if pressure is lost.
For ergonomic reasons, manual clamps are reserved for low-volume applications. Manual clamps are also used for applications in which parts must be held together for a long time, such as fixturing parts while an adhesive cures.
In-line clamps push or pull the workpiece. Hold-down clamps apply downward pressure on the workpiece, and are available in open-arm or solid-arm versions. Open-arm, or U-bar, clamps allow the spindle to be moved anywhere along the work-holding bar. In solid-arm clamps, the spindle is permanently in one place.
There are several types of clamp mechanisms. Toggle action clamps operate through a system of levers and pivots, explains Douglas W. Ruffley, PE, chief engineer at De-Sta-Co (Madison Heights, MI). Once the spindle contacts the workpiece, clamping force is exerted as the linkage elements stretch or compress. The mechanism positively locks when the center pivot moves past the centerline of the other two pivots and contacts a stop. The mechanism will remain in this locked position even in the absence of air or fluid pressure. The clamp unlocks only when the cylinder powers the linkage back through the locked position.
"That’s important, especially if you’re using the clamp to transport parts over a manned station," says Doberstein.
Cam-action clamps use frictional force to effect a locking condition between the cam on the bar and the follower on the handle. The cam action applies gentle pressure for gripping fragile materials, such as glass and plastic. It also enables the clamp to accommodate variations in workpiece thickness.
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Swing cylinder clamps have a clamping arm that rotates 90 degrees, then down. The swing of the clamping arm minimizes obstruction in the workspace and makes loading and unloading of parts easy. Swing cylinder clamps accommodate wide variation in part sizes, and they are ideal for welding or dirty environments, because the piston retracts to clamp and is protected during the process.
Several variables must be considered when specifying clamps. Holding capacity is the maximum external force that the clamp can resist in its locked position without incurring permanent deformation. "As the clamp point moves out from the base toward the end of the bar, the holding capacity decreases," says Ruffley.
Clamping, or exerting, force is the amount of force that is actually applied to the workpiece by closing and locking the clamp. This force will vary depending on the geometry of the linkage, the force applied to the clamp by the operator or cylinder, the workpiece material, the height and position of the spindle, and whether a cushion is attached to the spindle. "The clamping force should never exceed the holding capacity," Ruffley warns.
Clamping distance is the length between the body of the clamp and the point where the clamp grips the workpiece. Clamping range is the minimum and maximum thickness that the clamp can hold.
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