For instance, a typical slide with linear ball bearings is rated for a 20-lb overhung load with 0.005-in. deflection. Yet the unit's four bearings may have a combined load rating of several hundred pounds. This bearing overcapacity ensures precision and long life despite unfavorable loading conditions.

Carriage loads on guideshaft-style linear slides can deflect orbend the shafts, especially on longstrokes. Adding an optionalcenter support member oneach guideshaft dramaticallyincreases load capacity. Attaching shaft supports at the guide's center reduces deflection to less than 0.005 in., even with heavy loads. Rather than needing a larger, more-expensive unit, this lets designers use smaller slides with the same results.

Linear bearings, whether ball or sleeve, support the greatest loads when designs apply force over the entire length of the bearing. This is commonly known as a carriage load. Heavy loads, such as shuttling a toggle press or riveter between locations, are best handled with carriages. Carriageload slides with short strokes can carry several hundred pounds.

Cylinder bore and stroke also factor into load capacity. Obviously, a linear slide would be of no use if the cylinder couldn't produce enough thrust to move the load. By the same token, long-stroke slides with undersized guideshafts would not have the strength to be of any practical value. Pre-engineered linear slides account for these considerations and balance load capacity, available force, and structural integrity.

Stroke. Slides commonly come in 1.0-in.-stroke increments. Designers generally specify slightly longer strokes than applications require and add adjustable stops. These stops include clamp collars, and threaded bolts and stop nuts, and offer repeatable stopping accuracy to ±0.001 in.

Operating speed. Speed is an often-overlooked aspect of linear slides. Engineers sometimes find it difficult to get accurate speed information, yet ignoring speed factors can have disastrous results.

A safe speed range for pneumatic linear slides without external stops is generally 6 to 8 ips. A 12-in. stroke in 2 sec is approximately 6 ips — approximate because acceleration and deceleration time are not taken into account. On short strokes, ignoring acceleration/deceleration produces misleading results. A 1-in. stroke in 0.16 sec means an average speed of 6 ips but, in reality, final speed is much higher because a good portion of travel time involves acceleration.

High speeds create severe impact forces when loads abruptly stop at the end of stroke. Urethane bumpers mounted inside or outside the cylinder can cushion these forces if accuracy is not an issue. Adjustable stops with hydraulic shock absorbers or optional internal-cylinder air cushions provide more-precise, cushioned stops.

Speed is also related to the bearings. High speeds are best handled by linear ball bearings that permit travel velocity to 100 ips. However, avoid ball bearings with short-stroke, fast-reciprocating motions. Inertia of the ball circuit tends to make balls "skid" in their tracks when the direction suddenly reverses.

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