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Glossary

Submitted by eric@madebyjet… on Tue, 03/27/2018 - 19:46

Alignment

Alignment of the actuator, parallel to the line of motion, is critical. Also, the end effector connection must be designed to prevent any transfer of bending moments back to the actuator.


B10 Life

B10 Life is expressed in total revolutions or inches of travel a system will operate under a rated load. 90% of all systems operated at this rated load will meet or exceed this rating. Although 10% may not reach a million inches, 50% could exceed 5 million inches.

Life under load (B10 life) is predictable; severe load applications can generally be compensated for by providing additional capacity.


Contamination

Contamination of the ball screw system is the leading cause of premature failure. Providing a continuous, low pressure, air purge to the system is a good way to ensure clean operation.


Force

Force,  when calculating the required force, consider the force to accelerate the mass as well as the force to overcome the friction and the applied force. For sizing the system, consider the maximum force and duration. For evaluating life under varying loads, calculate the root mean cube equivalent load which weights the different load levels by the typical length traveled under that load. 


Impact

Impact is unacceptable to ball screws as well as anti-friction bearings. Severely shortened life and/or catastrophic failure are the results. Avoid impact or provide a mechanical system to buffer the ball screw assembly from shock loads.


Linear Velocity

Linear velocity is limited by (1): the maximum ball screw rpm without "whipping" of the ball screw shaft; (2) critical speeds for the ball nut assembly (beyond which the motion of the balls becomes erratic and performance life suffers).


Lubrication

Good lubrication is essential. Do not mix lubricants. Refer to the Service Manual to determine the correct lubricant. Important: Oil systems are shipped dry. Lubrication lines must be connected, and the reservoir and all connections filled prior to operating the actuator. Grease systems come pre-lubricated but should be periodically inspected (if applicable) to ensure adequate lubricant to critical areas of the ball screw.


Maximum Acceleration

Maximum acceleration of ball screw assembly is approximately 32 ft/s2, above this level, unit life becomes shorter and less predictable.


Side Loads

Side loads are undesirable. Almost any force not coaxial to the actuator compromises potential life. Isolate the actuator from all bending moments or at least recognize and minimize the amount of side loading.


Stiffness

System stiffness is an essential attribute of high accuracy positioning systems. Specifying 0.0002-inch positioning accuracy and repeatability without considering spring rate may result in disappointing performance. For precision light duty applications a spring constant of 1 x 106 lbs/in is acceptable and a range of 3 x 106 to 6 x 106 lbs/in may be necessary for higher load applications.

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