The contact angle of a ball nut describes the direction in which force is transmitted through each steel ball between the screw-shaft raceway and the nut raceway. In a conventional ball screw, this angle is commonly designed at approximately 45°, although the exact value depends on the raceway geometry, ball size, internal clearance and preload design.
Short answer: A ball nut does not normally use a 0° radial contact arrangement. A ball screw is primarily an axial-force transmission component, and its helical raceways create an angular contact between the balls, screw shaft and nut.
How Is the Ball Nut Contact Angle Defined?
At each loaded ball, a line can be drawn through the contact points between the ball and the two raceways. The angle between this load line and a reference plane perpendicular to the screw axis is called the contact angle. It indicates how the contact force is divided into axial and radial components inside the ball screw assembly.
Because a ball screw converts rotary motion into linear motion, its useful external load is mainly axial. The screw shaft, nut and balls work together as one assembly; the nut should not be evaluated as if it were a separate radial bearing.
Why Is Approximately 45° Common?
A nominal contact angle near 45° provides a practical balance between axial load transmission, stiffness, rolling behavior and manufacturability. However, "45°" should be understood as a typical design value rather than a universal measured value for every ball nut.
| Design factor | Relationship to contact | Practical effect |
|---|---|---|
| Raceway profile | Defines the nominal ball-to-raceway contact geometry | Influences load distribution, stiffness and contact stress |
| Ball diameter | Affects clearance or interference within the raceways | Can change preload, torque and the effective contact condition |
| Preload method | Establishes controlled contact before the external load is applied | Reduces axial clearance and increases rigidity, but also raises torque and heat |
| Manufacturing accuracy | Controls the consistency of raceway and ball contact | Affects torque variation, load sharing, noise and service life |
Contact Angle Is Not the Same as Lead Angle
These two angles describe different parts of a ball screw:
- Contact angle describes the load-transmission direction at the ball-to-raceway contact points.
- Lead angle describes the inclination of the helical thread and is determined by the lead and effective screw diameter.
Lead angle has a direct relationship with linear travel per revolution and affects drive torque and backdriving behavior. Contact angle is part of the internal raceway design. A large-lead ball screw therefore does not automatically have a larger contact angle.
How Does Preload Affect the Contact Condition?
Preload removes or reduces axial clearance by establishing internal force between the balls and raceways. Depending on the design, preload may be created by a double-nut arrangement, an offset-pitch single nut or selected oversize balls.
The important purchasing specification is normally the required preload or axial clearance-not a customer-selected contact angle. More preload can improve positioning rigidity and reduce backlash, but excessive preload increases friction torque, operating temperature and wear. It should therefore be matched to load, speed, duty cycle and accuracy requirements.
For more information on how the balls are arranged inside the nut, see the number of ball circuits in a ball nut.
Do Buyers Need to Specify the Contact Angle?
For most standard ball screw orders, no. The contact geometry is already determined by the manufacturer's screw, nut, raceway and ball design. Buyers should provide the operating requirements that affect ball screw selection:
- axial working load and peak load;
- required travel, speed and acceleration;
- positioning accuracy and acceptable axial clearance;
- duty cycle and target service life;
- installation orientation and end-support arrangement;
- lubrication and environmental conditions.
These values allow the supplier to check diameter, lead, nut style, load rating, permissible speed, accuracy grade and preload. The complete ball screw assembly should be selected as a system rather than choosing a nut from contact angle alone.
Do Not Confuse It with the Support Bearing Contact Angle
The angular-contact bearings installed at the fixed end of a ball screw have their own contact-angle specification. That value belongs to the support bearing, not to the ball nut raceway. Support bearings are selected and arranged to carry axial force, locate the screw shaft and provide the required support rigidity.
When checking a drawing or catalog, first identify whether a stated angle refers to the ball nut's internal raceway contact or to the angular-contact bearing in the support unit. Mixing these two specifications can lead to an incorrect selection.
Contact DLY for Ball Screw Selection
Send us your required diameter and lead, stroke, load, speed, accuracy, axial clearance and shaft-end drawing. DLY can help evaluate a suitable ball screw and ball nut configuration for your equipment.
Contact Us
