All You Neeed to Know About Ballscrews

1.    What are ballscrews used for?

Ballscrews are a method of movement using the rotational motion of ballbearings within a captive nut and transferring it to a linear motion without creating heat or friction.  Ballscrews are used for accurate movement side to side, up and down and in and out on computerised machinery.  They are used in many industrial manufacturing processes such as the automotive industry and aviation.  You will also find ballscrews in sliding doors and measuring equipment where accuracy is key.  In some applications ballscrews are performing multiple procedures in several axis at the same time to manufacture complex components

2.    What is the difference between a lead screw and a ballscrew?

A lead screw is only two components of which these parts could be made of different materials such as steel and phosphor-bronze, nylon or other low coefficient materials so that friction is not created and temperature is kept low.  The two component system is not as accurate and can be used in manufacturing processes where accuracy is not key such as food and drilling procedures.  A lead screw can either have a V form thread or a castellated thread.
A ballscrew, however, has multiple components such as transfers, wipers, tubes, balls, nut and shaft.  It transfers it load using recirculating ball bearings as part of a nut  Some units have these ballbearings passing through a tube transfer, others use an end cap transfer, others use internal transfer mechanisms where the balls do not pass out of the nut housing.  This method of using ball bearings minimises friction during movement and gives the ability to have higher helixes which gives faster travel with accuracy.

3.    How does a ballscrew actuator work?

A ballscrew actuator is a good replacement where hydraulic or air ram movement is not possible, turning rotational force into a piston like movement, extending and retracting a ram system.  This can be used in the aviation industry where oil is not possible, or the food industry where air rams have insufficient capacity.  By increasing the lead of the ballscrew, you can speed up or slow down the procedure.

4.    How do you pick a ballscrew?

Ballscrews are designed for a specific application as each requirement produces different loads or forces of resistance in its application.  These procedures could be lifting a mass, compressing liquid material, or the resistance created by the operating procedure that the machine tool is required to do.  Some procedures will have high force such as compressing molten plastic into a mould or a grinding procedure where the pressure has to be minimal but to be held accurately. Ballscrews come in many sizes from a few centimetres to several metres long.

5.    What size ballscrew do I need?

The ballscrew requirement for each application would be calculated by the forces which are apparent in the application, so these forces are created by the speed of movement, the weight of the component in motion and the feed rate required for machining material.  To select a ballscrew, the mathematical needs of the process with moving mass in the X, Y and Z axis would have to be calculated which would give you the correct ballscrew for each procedure giving you multiple solutions to the design of the ballscrew

6.    How do you calculate ballscrew speed?

Ballscrew speed is calculated by multiplying the diameter of the ballscrew shaft by maximum speed of rotation of the shaft or nut. This is dependant on the application and the cycles of movement per minute.  Different manufacturing processes will have different requirements for speed.

7.    Are ballscrews self locking?

No.  Due to the nature of converting rotational pressure onto a spiral of ballbearings, this motion will only become locked if one of the components becomes damaged.  Ballscrews will free run due to the nature of the forces and the low friction design.

8.    What is lead on a ballscrew.

Lead is the linear distance travelled for each complete rotation of the screw or nut and pitch is the distance between screw threads.  These terms are often used interchangeably.  For example, a lead of 10mm would mean that in 1 rotation of travel of the component has travelled 10mm.  The amount of movement along its length during 1 revolution of the nut or shaft

9.    How is ballscrew torque calculated?

Torque is the force required to turn the screw or nut.  This force can be increased depending upon the pre-load which is required by the nut.  Pre-load means that there is no movement between the two components (nut and shaft).  Ballscrews are designed to have a higher torque when manufactured knowing that the pre-load will drop during use as the ball track becomes worn. The requirement of torque is dependant on application

10.    What is backlash in a ballscrew?

Backlash is the unwanted movement between the two components, nut and shaft.  This is created by wear of the parts and needs to be eliminated in high precision applications.  Presence of backlash means that the movement is not immediate when the shaft or nut is rotated so this movement can be compounded in multiple sequences producing inaccurate procedures. A ball nut with all the backlash removed is called pre-loaded.

11.    What is ballscrew in CNC machine?

A CNC machine has a computer control which is translated into a numerical calculation to control the movement of the head or procedure that the machine is being asked to do in the X, Y or Z axis. In some machines these axis can be multiple.   When checking a machine for its accuracy it should be possible to move the tool or head in a perfect circle in each of the axis.  Ballscrews in this type of machine give the accuracy required for high precision applications such as the aero and automotive manufacturing industries

12.    What is the purpose of recirculation ballscrew nut in CNC?

The Rolling friction achieved in the recirculation is less than the sliding friction of a lead screw.  This then achieves a higher efficiency of power transfer which then increases the accuracy of the procedure and speed of the operation required

13.    What material are ballscrews made of?

Ballscrews are made from metal, usually steel.  However, the ball nuts may be made from other materials such as plastic.  The shafts are always machined from steel or stainless steel.  The balls within the nuts are generally steel, but in some applications ceramic balls are used.