Linear Slides

Page 7 Specifications subject to change without notice or incurring obligations – Engineering Considerations for Product Selection 1-12-10 Moisture Environments – Application Tips Coolant splash, water spray, and humidity applications can be handled by several methods. Duralon® and pre- chrome stainless steel shafting can be used. When linear ball bearings are used, Fabco-Air can supply units greased with a special moisture displacing lubricant and corrosion resistant plated guide shafts. Operating Speed Considerations An often overlooked aspect in the selection of linear slides is the speed at which it will operate. It can be difficult to obtain true and accurate speed information, yet ignoring speed factors can have disastrous results. Safe speed range is generally 6 to 8 inches per second if no external stop options are utilized. A 12" stroke in 2 seconds is approximately 6"/second speed. It is approximate speed because we have not taken into account acceleration and deceleration time. On shorter strokes, ignoring acceleration/deceleration can be very misleading. A 1" stroke in 0.16 second is an average speed of 6"/second, but in reality, mid-stroke speed is much higher because a good portion of time was accelerating up to speed. It then requires a higher speed to travel that same distance in the 0.16 second time span. This higher speed develops severe impact forces when it suddenly stops at the end of stroke. Machine cycle speed can also be misleading. Cycling at 30 parts per minute is a comfortable speed for moderate strokes. But, is the slide reciprocating at a uniform speed, or does it dwell (remain stationary) for part of the cycle? If so, the slide has to operate at a higher speed to make up for the time lost during dwell. High speeds can be handled safely and reliably with the right combination of bearings/shaft, adjustable stops, and bumpers or hydraulic shock absorbers. Here are some tips: Handling High Speeds High speeds are best accomplished using linear ball bear- ings, as they can handle speeds up to 100 inches of travel per second. One exception is on short stroke (less than 1"), high cycle applications. Short, fast reciprocating motion tends to make the recirculating balls skid on the guide shaft when direction is reversed quickly, due to the inertia of the balls travelling in their track. A sleeve bearing may be superior in those applications. Fabco-Air offers both linear ball and sleeve bearings on most slide and pick & place models. High speeds can cause heat buildup in the air cylinder caused by the friction of the seals. To minimize friction, most Fabco-Air slides are equipped with high quality, 80 durometer nitrile (Buna N) lip-type seals. Slides can be operated non-lubricated, but life expectancy is increased on high speed applications by using lubricated air. High speed can cause damaging impact forces when the slide suddenly stops at the end of stroke. Adjustable stops should be used wherever possible to absorb impact externally rather than allowing the piston to bottom out inside the cylinder. “TS200” models and larger as well as “L & S 500” models and larger are available with air cushions to help decelerate the slide near end of stroke. Also, most slide models are available with either urethane bumpers or hydraulic shock absorbers. Urethane bumpers are an inexpensive way to absorb moderate impact forces while providing quieter operations. Precision end of stroke stop positioning is not possible though, as allowance must be made for the urethane to deform. High loads at high speeds are best decelerated using hy- draulic shock absorbers. Hydraulic shocks can be sized to the application, and provide a reliable way of decelerating a load over a given distance, bringing the motion to a safe, smooth stop in much the same way that a car is braked to a uniform stop (linear motion energy is converted to heat and dissipated). Hydraulic shocks are used in conjunc- tion with adjustable stops. End of stroke stop positioning is precise (within .001") and pistons are not bottomed out in the cylinder. Linear energy remaining at the end of stroke after the hydraulic shock has decelerated the load is absorbed safely by the adjustable stop in the form of a minor impact force. With proper shock sizing, moderate to heavy loads can be operated at speeds up to 24 inches per second – and lighter loads even faster.