Low Cost Automation Tutorial

#133 Mastering Ball Screws - 3 : Ball Screw Operating Performances

Category : Linear Motion Components

October19, 2012

Feed screw is a mechanical element used to convert rotational motion to linear motion, or to generate large forces. The types and required performances are as follows.

Feed screw	Lead screw	A feed screw with the same contacting arrangement as a surface contact slide beaing.
	Ball screw	A feed screw with the same arrangement as a point contacting rolling motion bearing.
	Hydrostatic screw	A feed screw that transmits forces in non-contacting arrangement via air or hydraulic fluids.

(1) Ball screw's operating performances

Ball screw operating performance is characterized by its very small friction resistances. This is due to its structure that transmit forces with rolling motion of the steel balls similar to ball bearings.

(1) Static friction torque

-	The static friction torque is a static friction resistance during the rotary motion as opposed to a linear motion static friction resistance.
-	It is thought that the ball screw's static friction torque occurs right when the feed screw begins to turn where the steel balls slightly slide instead of roll.
-	This sliding of steel balls and the screw groove that generate the static friction torque mostly causes the ball screw wear.
-	The static friction torque differs depending on the rotational direction (For./Rev.) of the screw shaft.
-	Also, it differs with the position of ball recirculation paths (Positioned low or high, etc.).

(2) Dynamic friction torque

-	This is a rotational dynamic friction torque as opposed to the linear motion dynamic friction torque.
-	This is a friction resistance in dynamic condition, and is affected by the amount of preloads determined by steel ball size in relation to the screw groove size, as well as the lubricant characteristics.
-	The dynamic friction torque will vary according to the screw shaft rotational speed.
-	In case of oversized ball arrangement, the dynamic friction torque dramatically increases depending on the oversize amount. (See [Fig.1])
-	In case of Double Nut arrangement, as opposed to the Single Nut arrangement, larger number of steel balls used and longer length of contacting screw groove reduces dynamic friction torque variations due to averaging effect. (See [Fig.1]b)

[Fig.1] Explanation on relationship between Nut Construction and Dynamic Friction Torque

(3) Friction torque variations

-	Friction torque variations affect motor's speed variations, thus the linear motion velocity irregularities and positioning accuracy variations can be caused.
-	Allowable torque variation rate is defined in JIS B 1192. ■Example A C3 grade ball screw with an effective thread length of 4000mm or less with reference torque of 0.2~0.4Nm is allowed a torque variation of ±45%.

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