Ball Screw Arbor Size: Understanding The Perfect Fit

Ball screw arbor size is an important consideration when working with ball screws and ball nuts. When ball nuts are ordered individually, they are typically shipped on arbors, and the process of transferring the ball nut from the arbor to the ball screw requires careful execution to prevent the bearing balls from falling out. The inside diameter of the arbor plays a crucial role in this process, and adjustments might be needed if it is too small to fit over the outside diameter of the journal. Proper orientation of the ball nut and return tubes is also essential for optimal performance. Additionally, the flange used with the ball nut should be securely fixed, and pinning or set screws are common methods to achieve this.

Ball nut arbor transfer process

When ball screws and nuts are ordered individually, the nut will need to be assembled to the screw. This is usually done after the ends are machined for the support bearings to fit on. Ball nuts are shipped on arbors to prevent the loss of bearing balls. When transferring the ball nut from the arbor to the ball screw, follow these steps:

• Remove any ball nut retainer from the arbor. Hold the arbor firmly end-to-end with the screw, ensuring the arbor end is centred on the screw shaft end.
• Slide the ball nut down the screw shaft and rotate counter to the thread until you feel the balls drop into the screw thread. Then, rotate with the screw thread until the ball nut completely clears the end of the screw shaft adjacent to the arbor.
• If the inside diameter of the arbor is too small to slip over the outside diameter of the journal, apply tape to the journal to bring the outside diameter up to the root diameter of the screw. This prevents the bearing balls from falling out of the ball nut. The ball nut can then be transferred across the taped journal onto the ball screw.
• To transfer the ball nut from the screw to the arbor, reverse the procedure.

Cautionary Advice:

• Extreme care must be taken to prevent the ball nut from sliding off the end of the screw shaft during installation and handling. Temporary stops can be made by wrapping tape around the shaft ball grooves at each end. Be sure to remove the tape and any residual adhesive after the ball screw assembly is properly installed.
• The removal of the arbor from the ball nut will result in the loss of the bearing balls. All of the bearing balls in a ball nut are matched, so if any balls are lost during this transfer, they must all be replaced.

Ball screw lubrication

The choice of lubricant depends on the application. Light loads and high speeds do well with machine oil or low-viscosity grease, while medium loads and speeds are better suited to NLGI 1 or 2 grease. High loads may require a grease with an EP2 additive to prevent the grease from breaking down under high pressure.

When lubricating a ball screw, it is important to keep the screws with a slight film of grease or oil at all times. The screw should feel slick to the touch but not dripping. Grease can be applied directly to the screw's threads near the root of the ball track, or pumped directly into the ball nut if there are lubrication holes. Oil can be applied by running the nut back and forth after applying it to the outer diameter of the screw.

It is important to note that dry lubricants, such as PTFE powder or graphite powder, should not be used with recirculating ball or roller bearings. While these lubricants are suitable for sliding motion, they do little to aid rolling friction.

For oil lubrication, the viscosity and application rate are key variables, which depend on temperature, load, and speed. Oil that is too viscous or applied in excess can raise the operating temperature, while oil that is not viscous enough or applied in insufficient amounts can increase friction and wear.

For grease lubrication, the National Lubricating Grease Institute (NLGI) identifies nine grades based on consistency, with lower grades flowing better and higher grades being firmer. Grades 1 through 4 are often used in rolling contact bearings, with Grade 2 being the most common.

Ball screw efficiency

The high efficiency of ball screw systems is due to the minimal frictional loss between the nut and the screw. The nut and the screw do not come into direct contact, as the balls rolling between the helical grooves provide the only contact between the two components. This results in an output efficiency of 70 to 95 percent for ball screw systems.

In contrast, acme screw systems have a lower efficiency of 20 to 40 percent due to the high frictional force between the nut and the screw thread. This requires more motor torque to achieve the same force as a ball screw system, leading to a lower force, speed, and duty cycle rating for linear actuators using acme screws.

The efficiency of a ball screw system also contributes to reduced physical wear and tear, making it more cost-effective in the long run. However, it is important to consider the potential safety concern of back driving with ball screws due to their low internal friction. A braking system may be required to hold the load in place when the actuator is not in operation.

Additionally, the price of a ball screw is typically higher than that of an acme screw, and the ball mechanism can be noisier. Despite these drawbacks, ball screw systems offer higher efficiency, increased force and speed capabilities, and extended lifespan compared to acme screw systems.

Ball screw orientation

Axial Forces and Buckling

The ball screw experiences the full load as an axial force, which can cause the screw to bend and eventually buckle. The buckling load depends on the screw's root diameter, unsupported length, and end bearing arrangement. A screw with a fixed-fixed bearing arrangement can withstand a higher buckling load than one with a fixed-free end mounting. To avoid excessive compressive forces, the fixed bearing should be placed at the top of the screw, which will put it in tension.

Back Driving

Ball screws can often back drive, depending on their friction, lead angle, and efficiency. The likelihood of back driving can be determined by calculating the back-driving torque and comparing it to the assembly's friction force. Lead screws tend to have a lower tendency to back drive due to their lower efficiency.

Lubrication

Gravity can hinder the lubrication of upper tracks and raceways in a vertical orientation. It is important to follow the manufacturer's lubrication guidelines, as they may recommend against using oil in vertical applications. Grease lubrication, for example, may be recommended due to its ability to reach all critical surfaces through specially metered pathways.

Contamination

Vertical applications have the benefit of liquid contamination draining away, reducing corrosion risks. However, fine particulates like fiberglass and ceramic powder are more likely to adhere to bearing surfaces and cause contamination build-up, increasing the risk of bearing entry. To mitigate this, consider using linear guides with front and side seals and an actuator with its own sealing or covering mechanism, preferably a full-contact seal.

Bearing Blocks and Guide Rails

To support pitch and yaw moment loads during acceleration and deceleration, it is recommended to use two bearing blocks on each guide rail in a vertical application. Using an actuator with two parallel guide rails and two bearing blocks each is ideal for handling roll moments caused by unevenly distributed loads or external forces.

In summary, when designing a vertical application using a ball screw, careful consideration must be given to the orientation of the assembly, lubrication, contamination, and the selection of bearing blocks and guide rails. These factors will impact the performance and longevity of the ball screw system.

Ball screw design considerations

Mounting and Pinning of Ball Nut Flange:

The flange, if used, should be permanently fixed to the nut. It is recommended to order the nut and flange factory-assembled to avoid disassembly issues. The preferred method of securing a flange to a nut is by using a pin or set screw parallel to the screw, intersecting the flange and nut mounting thread. Alternatively, the flange can be drilled and tapped radially for a set screw.

Ball Nut Orientation:

Proper ball nut orientation is crucial, especially when used in orientations other than vertical. The return tubes must be positioned correctly to optimize ball nut operation and prevent issues during installation and handling.

Transferring Ball Nuts from Shipping Arbor:

Ball nuts are typically shipped on arbors and need to be carefully transferred to the ball screw. The arbor is placed against the end of the screw thread, and the ball nut is rotated onto the screw. If the arbor's inside diameter is too small, tape can be applied to the journal to match the root diameter of the screw, preventing the bearing balls from falling out.

Lubrication:

Proper and frequent lubrication is vital to achieving the predicted service life of the ball screw assembly. Without lubrication, the ball screw life can be reduced by up to 90%. Standard lubrication practices should be followed, and light oil or grease is usually suitable.

Load Considerations:

The total weight of the load should not exceed the static load capacity of the ball screw. The load direction should be coaxial with the screw whenever possible, avoiding overturning or cocking type loads. The number of ball screws used and the distribution of weight among them also play a role in the applied load.

Design Life Objectives:

After determining the applied operating load, the design life objective, measured in inches of travel, should be calculated. This calculation varies between horizontal and vertical applications and involves factors such as stroke length, operating hours per day, working days per year, and expected design life.

Load-Life Relationship:

The load-life relationship for a ball screw is an inverse cube ratio. Reducing the load on the ball screw increases its life expectancy, while doubling the load significantly decreases its life. This relationship allows for predicting the life of the ball screw based on the applied load and design life objective.

Critical Speed:

Critical speed is the speed at which the nut or screw may experience severe vibrations. The maximum safe operating speed is typically 80% of the critical speed rating. It is influenced by factors such as the unsupported length of the screw, screw diameter, and type of end bearing supports.

Safety Considerations:

Safety is a crucial aspect of ball screw design. Factors such as misalignment, impact loading, lack of lubrication, contamination, and external damage to return circuits can lead to premature ball nut failure. Preventative measures include using multiple screws to support the load, nuts with independent ball recirculation circuits, and ball deflectors to prevent balls from exiting the nut.

Frequently asked questions