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Linear Motion

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.

Visit the full library of Tech Tips for additional products.

When should a zip chain/rigid chain actuator be considered over a traditional electric screw style actuator?

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.

Is there a speed limit advantage using a roller screw instead of a ball screw?

There is a significant speed limit advantage using a roller screw. The advantage is more than 3x using a Roller Screw. The permissible speed limit is the speed that a screw cannot reliably exceed at any time. It corresponds to the limiting speed of the rotation of rollers in a SR & ISR type Roller Screw or the recirculation system in the ball screw nut. The speed limit calculation for each type of screw is as follows:      
Planetary Roller Screw:  n x d0 < 160,000 
Ball Screw (recirculation by inserts/tubes):  n x d0 < 50,000
n = rotational speed (rpm)
d0 = screw shaft nominal diameter (mm) 

Is there a drop-in replacement for ball or roller screws?

There is no industry standard for ball & roller screws. Interchanges are based on functional performance and not dimensions (no drop-in replacement). Key characteristics include nominal diameter x lead, dynamic and static capacity, speed, and life requirements.

Ball screws come in two general types: Rolled Thread and Precision Ground. What are the standard lead errors for each screw type?

Rolled Thread Screws:
Lowest Grade 0.018in./ft.
Standard Grade (most common) 0.090 in./ft.
Precision Rolled Grade 0.003 in./ft.
Precision Ground Thread Screws Error:
Class 3 0.001 in./ft.
Class 5 0.0005 in./ft.
Class 7 0.0002 in.ft.

Motion Control Made Simple, Mechanical Products: Drive Screws and Belts, Page 67

Why use a keyed screw jack?

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.

Why does grease separate?

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. 

What is the purpose of a wiper on a ball nut?

Wipers are nylon brushes installed in the ball nut which are designed to brush large particulates off of the screw. Debris which enters the ball nut can lead to increased wear or failure of the ball nut. A boot or bellows is recommended for harsh environments.

What is meant by lead screw accuracy?

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.

The Importance of Selecting the Correct Shaft Tolerance for Your Linear Bearing

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.

Maximize your Linear Bearing Life and Load Capacity

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.

Determining the Lead of a Ballscrew

Lead is the linear distance the nut or shaft moves in one full 360 degree revolution of the nut or the shaft. The best way to identify the lead is to first mount a dial indicator on the front face of the ballscrew nut. Mark the start point on the ballscrew nut. Rotate the ballscrew nut one full revolution and measure the linear distance travelled by the nut. This will be the lead of the ballscrew.

The information provided in Tech Tips is not meant to be all-encompassing, but rather to draw attention to and provide information about the particular subjects covered. All suggestions and recommendations contained in Tech Tips are based upon information that is believed to be accurate to the best of the experience and knowledge of PTDA’s contributing members, but are made without guarantee or representation as to results. PTDA and Tech Tip contributors expressly disclaim any warranties or guaranties, express or implied, as to the accuracy or completeness of any information published in Tech Tips, and disclaims and makes no warranty that the information in Tech Tips will fulfill any of your particular purposes or needs. PTDA and Tech Tip contributors disclaim liability for any personal injury, property, or other damages of any nature whatsoever, whether special, indirect, consequential, or compensatory, directly or indirectly resulting from the publication, use of, application, or reliance on Tech Tips.

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