What Determines The Load of A Ball Screw?

The load of the ball screw is determined by several factors:

1. Basic dimensions of the screw

Nominal diameter: The larger the diameter, the greater the cross-sectional area of the screw, and the greater the axial and radial forces it can withstand, resulting in stronger carrying capacity. For example, a screw with a diameter of 50mm usually has a higher carrying capacity than a screw with a diameter of 25mm.

Pitch: The pitch affects the transmission efficiency and speed of the screw. A larger pitch results in a faster linear motion speed, but the accuracy may be relatively lower; a smaller pitch provides higher accuracy, but the carrying capacity may be limited. A balance needs to be struck between the pitch and the load capacity based on specific application requirements.

2. Ball Parameters

Ball Size: The larger the diameter of the ball, the greater the pressure it can withstand. Larger balls can disperse larger axial forces, enhancing the load-bearing capacity of the screw.

Number of Balls: The more balls there are, the more evenly the load is distributed among the balls, effectively improving the overall load-bearing capacity of the screw. More balls can share the axial force, reducing the force on a single ball.

3. Material Properties

Strength and Hardness: The strength and hardness of the material used for the screw and nut directly affect the carrying capacity. High-strength alloy steel, carburized steel, etc. have higher yield strength and fatigue resistance, and can withstand greater loads. Compared to ordinary carbon steel, they have stronger carrying capacity.

Elastic Modulus: The higher the elastic modulus of the material, the smaller the deformation of the screw when bearing loads, which can better maintain accuracy and stability, and indirectly affects the carrying capacity.

4. Pre-tightening Force and Contact Angle

Pre-tightening Force: An appropriate pre-tightening force can eliminate the gap between the screw rod and the nut, improving the transmission accuracy and rigidity, and enhancing the load-bearing capacity. Excessive pre-tightening force may lead to increased friction and accelerated wear; insufficient pre-tightening force will not effectively utilize the load-bearing potential of the screw rod.

Contact Angle: The contact angle between the ball and the raceway affects the efficiency of axial force transmission. A suitable contact angle enables the ball to better withstand axial force, thereby improving the load-bearing capacity. Generally, the contact angle is between 30° and 45°, and it can be selected based on the load type and application requirements.

5. Installation Method and Supporting Conditions

Installation Form: Different installation methods (such as fixed-fixed, fixed-support, support-support, etc.) have a significant impact on the bearing capacity of the screw rod. The fixed-fixed installation method can withstand large axial and radial forces, and is suitable for high-load and long-stroke applications; the fixed-support method is suitable for medium-load and precision requirements scenarios.

Supporting Rigidity: The higher the rigidity of the supporting structure of the screw rod, the better it can transfer the load, reduce the bending deformation of the screw rod, and improve the bearing capacity. The type and precision of the supporting bearings also affect the bearing performance of the screw rod.

6. Lubrication and Friction Conditions

Lubrication effect: Good lubrication can reduce the friction between the ball and the raceway, decrease energy loss, and enhance the carrying capacity. The type, viscosity, and lubrication method (such as grease lubrication, oil mist lubrication, etc.) of the lubricant need to be selected based on the load size, rotational speed, and working environment.

Friction coefficient: The smaller the friction coefficient, the less energy loss when the screw rod transmits the load, and the relative carrying capacity increases. Factors such as the surface roughness of the material and the lubrication state will affect the friction coefficient.

7. Working Environment and Load Type

Temperature: High temperatures may cause the performance of the screw rod material to decline, reducing its carrying capacity; low temperatures may make the material brittle, increasing the risk of fracture. It is necessary to select appropriate materials and lubrication methods based on the working environment temperature.

Vibration and Impact: Frequent vibration or impact loads will accelerate the wear and fatigue damage of the screw rod, reducing its carrying capacity. Measures such as vibration reduction or the selection of impact-resistant screws should be taken.

Load Direction: Unidirectional loads and bidirectional loads have different force conditions on the screw rod. Bidirectional loads (such as alternating tension and compression) have higher requirements for the strength and rigidity of the screw rod, and appropriate screw specifications and pre-tightening methods should be selected.

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