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Enhance your employees’ product and industry knowledge with PTDA’s Tech Tips. This library of online tips, hints and techniques may be used to educate new and current employees on power transmission/motion control (PT/MC) products, technologies and concepts and serve as reference and reminder for more experienced staff. Tech Tips are based on the expertise of PTDA member companies and content from PTDA’s Power Transmission Handbook®, the definitive resource and training tool on PT/MC products.
The number one reason to use a zip/rigid chain actuator over a more traditional style is speed. Because the zip/rigid chain actuators use a chain/sprocket combination instead of ball/machine screw, the contact forces are much lower and the unit can operate at a much higher speed. The second major reason to choose a zip/rigid chain actuator over ball/machine screw is if you have a limited installation footprint. Because a zip/rigid chain actuator uses chain and not a solid rod, the chain is able to be “un-zipped” and stored in a compact body that is a fraction of the size of traditional actuators.
A keyed screw jack is used any time the screw is unable to be restrained from rotation. This could be caused by an unguided load or when the screw must move through space before contacting the load. To restrain the screw from rotating, a keyway is machined down the screw length and a mating key is fixed inside the jack to prevent screw rotation.
When grease-lubricated power transmission components such as a screw jack is not operated for a period of time, the oil in the grease can come out of suspension from the grease's thickener. This oil can then escape the jack and is known as "oil bleed." You can prevent this by operating the jack periodically to keep the oil suspended in the thickener.
The accuracy of a screw is referred to as lead error. Lead error is the deviation from the actual measured screw lead compared to the theoretical mathematical screw lead and is often expressed in inches per foot cumulative.
Selecting the correct shaft tolerance range for round linear bearing applications is essential. The shaft (which acts as the inner race) must not only meet the linear bearing manufacturer’s specifications for hardness, roundness, straightness, surface finish and depth of hardness, but also have the correct OD tolerance to achieve the optimum running condition without being subjected to any unintentional preload or excessive play. The tolerance “class” you select for the shaft should also conform to the manufacturer’s recommended tolerance based on the tolerance of the bearing’s ID. Selecting a shaft with a lower than specified tolerance will result in excess radial clearance and a reduction of precision and position repeatability in the application; conversely, an oversized shaft may result in an unwanted preload condition which will result in increased rolling resistance, premature wear and excessive heat.
The orientation of your linear bearing in relation to the direction of your primary load can have a dramatic effect on the performance and life of your linear system. To achieve the maximum dynamic load ratings the ball circuits should be positioned to share the load as equally as possible. It is best to determine the direction the greatest amount of load is likely to be applied and then ensure that two ball circuits are sharing the load evenly rather than one ball circuit doing a majority of the work.