Tech Tips - Selecting Pneumatic Linear Slides for Automation Projects
sponsored by: 5 Heavy loads, such as shuttling a toggle press or a rivet unit between two work locations, would best be handled with a linear slide configured as a carriage. Note that a carriage load slide with a short stroke can carry several hundred pounds of workload. Another element factored into a linear slide’s load capacity is the bore and stroke. Obviously, a linear slide with a load capacity of 200 pounds would be of no use if it were powered by a 1/2” bore cylinder that produced only 10 or 15 pounds of thrust. By the same token, a 30” stroke slide with small 1/4” diameter guideshafts would not have sufficient strength to be of any practical value. Pre-Engineered linear slide packages take into account the need to balance out load capacity versus force available. Stroke Slides are commonly offered in 1.0” stroke increments. Designers generally will specify a stroke slightly longer than the application requires, and utilize optional adjustable stops. These hard stops will repeat within +/- .001” stopping accuracy. Stop options may consist of clamp collars or a threaded stop bolt / stop nut arrangement shown here in Figure 3. slide selection - continued Flange Nuts Threaded Rod Toolbar Rear Stop Bar Figure 3 Linear slide operating speed An often over looked aspect is speed. It can be difficult to obtain accurate speed information, yet ignoring speed factors can have disastrous results. A safe speed range for a pneumatic linear slide without external stops is generally 6 to 8 inches per second. A 12” stroke in 2 seconds is approximately 6” per second. It is approximate speed because acceleration and deceleration time has not been taken into account. On shorter strokes, ignoring acceleration/deceleration can be very misleading. A 1.0” stroke in 0.16 seconds is an average speed of 6” per second, but in reality, final speed is much higher because a good portion of time was spent accelerating. High speeds develop severe impact forces when stopped suddenly at end of stroke. Rubber bumpers, either within the cylinder or external, can be used to cushion these forces if stopping accuracy is not an issue. Adjustable stops in conjunction with either hydraulic shock absorbers or optional internal cylinder air cushions can be employed to provide a cushioned, precision stop. Bearing type and speed are also related. High speeds are best accomplished with linear ball bearings, which can travel up to 100” per second. Short stroke, fast-reciprocating motions should avoid ball bearings. The inertia of the ball circuit tends to make the balls “skid” in their tracks when direction is reversed suddenly.