Friction is one factor in ball screw performance. There are 2 different friction torques affecting screw operation performance. Each friction torque is dependent on the type of feed screw.
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 bearing.|
|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.|
Ball screw’s operating performance is characterized by its very small friction resistances. This is due to its structure that transmits forces with rolling motion of the steel balls similar to ball bearings. When given the load and lead screw dimensions, the following formula gives you the torque generated.
L = Ball Screw Lead
P = Load
(1) Static Friction Torque
The static friction torque is a static friction resistance during rotary motion. 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 in the screw groove is what generates the static friction causing the ball screws to wear. The static friction torque differs depending on the rotational direction (forward or reverse) of the screw shaft. It also differs with the position of ball re-circulation paths (positioned low or high, etc.).
(2) Dynamic Friction Torque
Dynamic friction torque is the 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 lubricant characteristics. It will also vary according to the screw shaft rotational speed. In the case of oversized ball arrangement, the dynamic friction torque dramatically increases depending on the oversize amount, (See Figure 1a). In the case of double 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 Figure 1b).
(3) Friction Torque Variations
Friction torque variations affects motor’s speed variations, therefore linear motion velocity irregularities and positioning accuracy variations can be caused. Allowable torque variation rate is defined in JIS B 1192. For example, a C3 grade ball screw with an effective thread length of 4000mm or less with reference torque of 0.2~0.4 Nm is allowed a torque variation of ±45%. This is a guideline to follow when specifying ball screws in applications and designs.
Thus far, ball screw characteristics, preloads and performance have been reviewed, up next the optimization of ball screw speed is examined, stay tuned!