How To Select Ball Screw

As we know, ball screws are used in automatic machines more and more, especially CNC machines and machining centers. So, correct ball screw could increase precision and production capacity. But, there are so many types, how to choose? Let’s analyze it together.

1.Decision conditions
General speaking, before you select ball screws, you need to consider below conditions firstly: load, feed speed, running mode, screw shaft speed, the stroke length, installation direction(horizontal or vertical), life time, positional accuracy.

2.Preselected ball screw specifications
According to above conditions, accuracy grade of ground ball screw, axle diameter of screw, screw pitch and overall length are preselected.

• Accuracy grade
C0~C10 is JIS standard, general machines use C7/C10, CNC machines use C3/C5, aviation machines use C0/C1, so, the smaller the number, the higher the accuracy.

• Axle diameter
Load decide diameter. Horizontal: the maximum axial load is subjected to acceleration and minimum load during deceleration. Vertical: the maximum axial load is borne when rising, and the minimum load is borne when it falls. Therefore, the smaller of screw’s axle diameter, the allowable axial load of the screw is smaller.

• Screw pitch
The two adjacent threads correspond to the axial distance between the two points on the mother line of the middle diameter cylinder. General, the larger the pitch, the larger the lead. Meanwhile, large lead is suitable for fast moving.

• Length
Over all length = stroke length + nut length + safe margin + installation length + connection length + margin
Sometimes, you add protection, such as a sheath, you need to take into account the scaling ratio of the sheath (generally 1:8, the maximum elongation of the sheath divided by 8)

3.Check basic security

• Allowable axial load
The allowable axial load is the load which may possibly cause buckling of the screw axis. The maximum axial load applied to the screw axis must be less than allowable axial load to ensure its security.

• Allowable rotate speed
Ball screw’s rotate speed is according to necessary feed speed and screw pitch, and it must be less than allowable rotate speed.

• Life time
The working life is the total rotation times, time and distance that ball rolling surfaces or any ball get fatigue due to alternating stress, until the balls start to peel off. So, calculate the life time of screw axis could ensure the life time required.

4.Discuss the performance requirements
When ball screw is working, you hope linear ball screw improve the precision of precise positioning and responsiveness, the following confirmation is required.

• Rigidity of the screw axis
The rigidity of the screw system’s each part must be considered.

• Temperature and life time
The tissue of material will change when the ball screw is used for more than 100 degrees or a short period of time but when used in a very high temperature environment. And the basic dynamic load will decrease as the temperature increase. Usually, ball screw is used within -20℃~80℃. When in high temperature environment, the grease must be changed to heat resistant, and check heat-resisting temperature of each part.

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