What is a Ball Screw and how do Ball Screws Work?
A ball screw is a mechanical assembly composed of a screw and a nut linked by ball bearings that roll in the threads of the screw and the nut. The low rolling friction of the ball bearings makes ball screws highly efficient devices for converting rotary motion into linear motion. The ball bearings, screw threads, and nut threads are made of hardened steel which gives ball screws high load capacities. Commonly there are wipers at each end of a nut to prevent the ingress of debris.
There must be a ball return system built into the nut. The ball return system recirculates the ball bearings in the nut or else the balls would exit the nut out the end as it traveled along the screw.
The primary ball return system used by PBC Linear is a multiliner return system. The multiliner ball return system guides turns of balls back to the same threads creating ball circuits within the threads. The number of circuits in a ball screw nut refers to the number of loops of balls
What is the difference between a ball screw and a lead screw?
A ball screw is a mechanical assembly composed of a screw and a nut linked by ball bearings that roll in the threads of the screw and the nut. The low rolling friction of the ball bearings makes ball screws highly efficient devices for converting rotary motion into linear motion. The ball bearings, screw threads, and nut threads are made of hardened steel which gives ball screws high load capacities. Commonly there are wipers at each end of a nut to prevent the ingress of dirt and contaminants. There must be a ball return system built into the nut. The ball return system recirculates the ball bearings in the nut or else the balls would exit the nut out the end as it traveled along the screw. The threads of the screw and nut normally have a gothic arch profile to accommodate the bearing balls rolling in them.
A lead screw and it’s nut normally have some type of v-thread or even a square thread. The screw is commonly made of steel and the nut is commonly machined, molded, or cast from a solid piece of polymer or a soft metal like bronze. A lead screw relies on sliding of the screw’s threads and the nut’s threads against each other to convert rotary motion into linear motion. Because it relies on sliding friction between the screw threads and the nut threads, a lead screw is not as efficient as a ball screw which relies on rolling friction. The nuts for lead screws are available as solid plain nuts and in anti-backlash designs. Often a lead screw is given a special coating to reduce friction.
What are the advantages of ball screws vs. lead screws?
What types of ball screws are available?
Ball screws are available in different screw diameters and different leads. The lead of a ball screw is the distance that the nut travels linearly when it makes one rotation on the screw.
The available ball screw sizes from PBC Linear are listed below.
| Ball Screw Diameter | Lead |
|---|---|
| 6 mm | 1 mm |
| 6 mm | 2 mm |
| 8 mm | 1 mm |
| 8 mm | 2 mm |
| 8 mm | 2.5 mm |
| 10 mm | 2 mm |
The nuts for ball screws are available in many shapes so that they can be mounted in different ways. Two of the most common nut shapes are flanged and cylindrical. Those shapes are offered by PBC Linear.
Ball screws are available in different grades of lead accuracy. The common standards used to grade ball screws are JIS B 1192-1997 and ISO-3408-1,2,3,4,5.
These standards grade the accuracy of the ball screw leads, the runout and location tolerances of their journals, and the dynamic preload drag torque deviation of their nuts. PBC Linear offers ball screws of grades 5, 7, and 10 per the ISO-3408-3 standard’s criteria.
What materials are ball screws made from?
Ball screws and their nuts are typically made from high-strength alloy steel. Ball screws of stainless steel or with surface treatments to enhance durability and corrosion resistance are also available by special request. Standard PBC Linear ball screws are made from 1050 steel that is induction hardened to ensure high load capacity and long life.
What is backlash in a ball screw and how can it be reduced?
Backlash is the amount of free axial movement by the nut when the screw is held stationary. The axial movement is possible because of clearance between the nut threads, the bearing balls, and the screw threads.
The amount of thread clearance can be reduced in a nut by installing larger diameter bearing balls, using double-nut designs, and other techniques. In PBC Linear’s miniature ball nuts, backlash is reduced by installing larger diameter bearing balls to reduce the clearance in the threads.
Standard backlash is <= .050 mm or <=.002 inch.
Reduced backlash is <=.013mm or <=.0005 inch.
What is the static load rating of a ball screw, C0a?
The basic standard load rating, C0a, is the axial static load at which the maximum permanent deformation of the ball & raceway at a contact point between the ball and raceway is 1/10,000th of the ball diameter. This is the static axial load that a ball screw can support without damage that would impact its performance.
It is used for ball screws that are stationary or rotating very slowly (<10 rpm).
The maximum permissible static load, Fa max, can be calculated using the formula:
Configure & Quote
Fa max = C0a / fs
where: fs: static safety factor
| Machine Type | Conditions | (fs) Factor |
|---|---|---|
| General Machinery | No vibration or impacts | 1.0 to 2.0 |
| With vibration or impacts | 2.5 to 7.0 | |
| Machine Tools | No vibration or impacts | 1.0 to 1.5 |
| With vibration or impacts | to 3.0 |
What is the dynamic load rating of a ball screw, C0a?
The basic dynamic load rating, Ca, is the axial load at which 90% of ball screws should endure a life of one million (1,000,000) revolutions when properly lubricated and without varying loads, vibration, or impacts.
Can the nominal service life of ball screws be estimated?
Yes, the service life of ball screws can be estimated. The formula to estimate the service life that 90% of identical, properly lubricated ball screws are expected to reach is given below (if the application has no significant vibration or impacts and the axial load and speed are constant):
L10 = (Ca / Fm)³ × 10⁶ (revolutions)
Where:
To convert the estimated service life to hours, the following formula can be used:
Lh = L10 / (nm × 60) (hours)
Where:
How is the service life of the ball screw estimated if vibration or impact loading will occur?
In applications where load and speed are constant, but vibration or impact loading will occur, the estimated life can be calculated as follows:
L10 = (Ca / (fw × Fm))³ × 10⁶ (revolutions)
Where the range of the load derate factor (fw) is given in the table below:
| Vibration or Impact | (fw) Factor |
|---|---|
| Minor | 1.0 to 1.2 |
| Low | 1.2 to 1.5 |
| Moderate | 1.5 to 2.0 |
| High | 2.0 to 3.5 |
To convert the estimated service life to hours, the following formula can be used:
Lh = L10 / (nm × 60) (hours)
Where:
How is the service life of the ball screw estimated if the axial load will vary?
The average axial load, Fm, for constant rotational speed and varying axial load is given by the following formula:
Fm = ∛(F1³ × p1 / 100 + F2³ × p2 / 100 ⋯ + Fn³ × pn / 100) (N)
Where:
The service life estimation formula is:
L10 = (Ca / Fm)³ × 10⁶ (revolutions)
To convert the estimated service life to hours, the following formula can be used:
Lh = L10 / (nm × 60) (hours)
Where:
How is the service life of a ball screw estimated if the rotational speed will vary?
The average rotational speed is:
nm = n1 × P1 / 100 + n2 × P2 / 100 ⋯ + nn × Pn / 100 (rpm)
Where:
The service life estimation formula is:
L10 = (Ca / Fconst)³ × 10⁶ (revolutions)
To convert the estimated service life to hours, the following formula can be used:
Lh = L10 / (nm × 60) (hours)
Where:
How is the service life of a ball screw estimated if the both the axial load and the rotational speed will vary?
The average axial load, Fm, for varying axial load is given by the following formula:
Fm = ∛(F1³ × p1 / 100 + F2³ × p2 / 100 ⋯ + Fn³ × pn / 100) (N)
Where:
The average rotational speed is given by the following formula:
nm = n1 × P1 / 100 + n2 × P2 / 100 ⋯ + nn × Pn / 100 (rpm)
Where:
The service life estimation formula is:
L10 = (Ca / Fm)³ × 10⁶ (revolutions)
To convert the estimated service life to hours, the following formula can be used:
Lh = L10 / (nm × 60) (hours)
Where:
14.What is the Dn value of a ball screw?
The Dn value divided by a ball screw’s outside diameter is the maximum speed allowed for the ball return system in the nut of a ball screw. The value is determined by the ball screw’s manufacturer.
For example, the maximum allowable speed of the ball return system in a nut with a Dn value of 50,000 rpm and an outside diameter of 8 mm would be:
nmax = 50,000 / 8 = 6,250 rpm
Note that the calculated maximum speed based on the Dn value is not the only check that must be made when verifying that a ball screw will work in an application. The critical speed of the ball screw must also be checked.
What is the critical speed of a ball screw?
The critical speed of a ball screw identifies the rotational speed at which the first natural frequency of vibration would occur in the ball screw. It is highly dependent on the ball screw’s diameter, length, and how it is supported. Ball screw manufacturers provide simplified formulas that account for the ball screw’s material and other factors.
For PBC linear ball screws, the formula is:
nmax = K × 10⁶ × d2 / (la²) × S.F.
Where:
What does it mean that an actuator will “back drive”?
An actuator back drives when the force applied to the carriage of an actuator overcomes the resistance of the drive train and causes the carriage to move. A common situation when this can occur is an actuator operating vertically with a weight load on the carriage. When power to the driving motor is removed, the weight on the carriage may overcome the resistance of the drive train, and the carriage may drop. In such situations, a designer may choose a low efficiency drive system that will not back drive or a brake may be installed in the drive train. The brake would be used to lock the system when power is removed from the actuator’s motor.
What is lead accuracy of a ball screw?
There are several lead accuracy criteria for ball screws called out by the ISO and JIS standards. The most common lead accuracy measurements in the ISO standard are the lead accuracy per 300mm of travel and the lead accuracy for 2π radians revolution. Of the two lead accuracy criteria, the lead accuracy per 300mm is most commonly used.
A table of permissible deviations over 300mm of travel is given below for different grades of ball screws:
| Standard Tolerance Grade | 0 | 1 | 3 | 5 | 7 | 10 |
|---|---|---|---|---|---|---|
| Permissible Travel Distance Deviation over 300mm | 3.5 μm | 6 μm | 12 μm | 23 μm | 52 μm | 210 μm |
What are ball screw grades?
Ball screws are available in different grades of permissible tolerances. The common standards used to grade ball screws are JIS B 1192-1997 and ISO-3408-1,2,3,4,5. These standards grade the accuracy of the ball screw leads, the runout and location tolerances of their journals, and the dynamic preload drag torque deviation.
There are several lead accuracy criteria for ball screws called out by the ISO and JIS standards. The lead accuracy criteria most frequently used in the ISO standard is the lead accuracy per 300mm of travel. A table of permissible deviations over 300mm travel is given below for different grades of ball screws:
| Standard Tolerance Grade | 0 | 1 | 3 | 5 | 7 | 10 |
|---|---|---|---|---|---|---|
| Permissible Travel Distance Deviation over 300mm | 3.5 μm | 6 μm | 12 μm | 23 μm | 52 μm | 210 μm |
The runout and location tolerances of journals are related to mounting accuracy. The criteria for dynamic preload drag torque only pertain to ball nuts that are preloaded. By checking the drag torque of the nut on a screw, the amount of preload can be verified.
Note that when grading ball screws, a lower grade number corresponds to better lead accuracy and tighter tolerances. PBC Linear offers ball screws of grades 5, 7, and 10 per the ISO-3408-3 standard’s criteria.
Can ball screws be repaired?
More expensive, large diameter, ball screws are sometimes repaired. Miniature ball screws are not often repaired.
Sometimes ball screws and the nuts are ordered separately, and the user must transfer the nut onto a screw. One must be especially careful transferring the nuts on or off miniature ball screws because the ball bearings used are so small. If balls are lost while transferring the nut of a miniature ball screw (or larger ball screw) onto a screw, the nut can often be returned to the manufacturer to have balls reinstalled.
In cases where balls are lost during transfer and the ball screw and its nut are to have reduced backlash or a preloaded fit, both the nut and screw will often have to be returned to the manufacturer to have new ball bearings installed.
Can ball screws be installed and used as received?
If ball screws are ordered with a specific lubricant called out, then they often can be used as received from the manufacturer. However, most ball screws ship from the manufacturer with only a rust inhibitor applied. The rust inhibitor is not a sufficient lubricant. Ball screws received with a rust inhibitor applied should be cleaned with a solvent and then a proper lubricant should be applied before the ball screw is used. Ball screw manufacturers often provide recommendations for general lubricants. Users may have to determine the optimum lubricant for their applications. Lubricant suppliers are often the best providers of lubricant recommendations.