Linear thrusters use double-acting pneumatic cylinders or hydraulic cylinders mated to shafts and plates to provide reoccurring linear motion. Linear thrusters typically use high air pressure (up to 250 psi) or low hydraulic fluid pressure (up to 500 psi) for their motive power. Motion is guided by two or four round shafts that are supported by linear bearings (usually a linear ball bushing). These products apply a thrust, or load, via a tooling plate mounted to the end of the shafts. They are not used for precision positioning applications. They may be used for lifting applications, or to apply thrust load.

Linear thrusters are often confused with linear slides or linear actuators; however they are different products and should be treated as such when searching for an application-specific product. The main differences between the three products is that linear slides are used for precision positioning applications, linear actuators are compact assemblies used for precision positioning applications and may be able to provide a thrust load (depending on the design), while linear thrusters only provide thrust. Linear thrusters consist of two or four shafts that are guided by linear bearings and are usually connected together by a flat tooling plate on the end of the shafts. A cylinder is used to move the tooling plate back and forth. Note that pneumatic and hydraulic cylinders are used rather than a motor because linear thrusters are meant to apply a load; cylinders will stop moving when the opposing force equals the applied force without any damage to the assembly but a motor would burn out in this application without sensors or other safeguards.

Parts of a linear thruster. Image Credit: Peninsular Cylinders

Proper care is important for maintaining functionality and work life span for linear thrusters. Oil is the best lubrication and several drops of oil should be added to the bearings after a set number of operational hours (see manufacturer notes for best lubrication practices of the device).

Bearing Types

Several bearing types are commonly available.

• Balls or roller bearings are used in applications that require reoccurring linear motion.
• Solid metal bushings use materials such as oil-impregnated bronze.
• Continuous-surface composite bearings are often recommended for their anti-corrosive properties and tolerance to debris.

Performance Specifications

Linear thrusters can be selected based on various performance specifications in order to select the best product for the application.

• Maximum stroke is the maximum distance that the shaft travels from the fully retracted position to the fully extended position.
• Maximum speed is rated at low or no load conditions. Flow control should be used for air or hydraulic cylinders to control velocities. If the movement of the piston is not controlled the cylinder and slide could be damaged by high acceleration rates and consequent high inertias.
• Operating pressure is the full required range of pressures in which the thruster will be operating.
• Operating force is the maximum force the liner thruster can provide. It varies with operation pressure and cylinder bore size.
• Operating temperature is the full-required range of temperatures in which the thruster will be operating.
• Bore size is the diameter of the cylinder which determines the maximum force that the cylinder can exert.
• Shaft diameter is the measurement of the shaft and is usually measured in inches (in).

Number of Guide Shafts

For linear thrusters with multiple guide shafts, suppliers specify the diameter of the largest shaft.

Number of shafts	Image
Two	Image Credit: Techno Inc.
Four +	Image Credit: Ultramation

Mounting Options

The integral stages of linear thrusters are mounted vertically or horizontally.

Vertically mounted - In order to properly mount a vertical unit, determine how fast the thruster must move and oversize the cylinder by increasing the system pressure or the bore size by the desired speed factor. The tooling plate can be a significant weight and should be included in the force calculations.

Horizontally mounted - In order to properly mount a horizontal unit, the required force can be calculated by multiplying the weight of the load by the coefficient of friction of the two sliding surfaces. Use the same rules for oversizing the cylinder using the calculated force.

Linear Thruster Features

Many linear thrusters include features such as:

Bumpers or cushions are used for heavy loads or where long stroke cylinders must be operated at high piston speeds. Cushions trap air into either or both ends of the main piston. This allows the main piston to undergo a controlled deceleration as it approaches the end of the cylinder.

Adjustable stroke- Adjustable stroke allows for the end points or the total stroke length to be adjusted.

Closed loop control- In a closed-loop system, operating parameters are monitored and compared to the desirable value. If there is a difference between the two, adjustments are made until the system is operating at the desired specifications.

Shock absorbers- Shock absorbers are used in pneumatic or hydraulic fluid absorption of shock.

Image credit:

Ultramation, Inc.