balls, but this reduces load capacity because there are less load balls. A synthetic plastic cage has been developed that adds many desirable properties to any recirculating ball bearing application. The plastic cage prevents ball-on-ball contact, reducing friction, jamming and increasing life. These cages also trap oil and grease helping to keep the ball well-oiled.
4. Seals: There are both standard and high-performance seals for use in different environments. All seals prevent foreign matter from entering the space between the nut and ball screw. Some seals are also for lubricant retention.
- Standard Seal:
- Plastic: used for general or standard ball screws and are of the non contacting type: blocking debris by means of obstruction
- Brush: used for rolled ball screws and are of the contacting type, brushing debris off the track
- High Performance: Seals that are made specifically for an individual screw nut and lead combination. These seals have a special lip that blocks contaminants, while only slightly increasing operational torque. These seals are used for especially dirty jobs, such as woodworking, welding, and automobile manufacturing.
What is Accuracy?
Accuracy is one of the crucial components in ball screw operation, as ball screws are utilized primarily for their precision and accuracy retention. Lead error and mounting error are two calculated error parameters that are important to consider in your ball screw application.
1. Lead Error: Lead Error is defined in two ways: the difference between the expected length (L) and the actual length (L+/-e), and the sum of all the e's (∑e=E ). For example, if the L is 5mm and the e=+/-.005mm the possible length could be 4.995-5.005mm.
2. Mounting Error: Mounting Error is caused by the way the screw is supported and the installation of the ball nut. The three most likely mounting errors include, bearing misalignment, coupling misalignment, and nut misalignment. Mounting error can create noise, shorten life, and cause positioning errors.
What is Preload?
Preload is a load that is put on a part (bolts, screws, bearings, etc) before its operating load is applied. This load causes some elastic deformation that improves performance. Elastic deformation is a change in shape that is fully reversible, like squishing a rubber ball. In the case of ball screws, the preload is on the interface of the nut and screw. The ball bearings and both the nut and screw grooves are all “squished” together.
This "squishing" eliminates backlash and reduces deflection. Backlash is the amount of lost motion caused by looseness in the drive train which can cause placement errors. The preload creates a tight fit for the nut, eliminating backlash. The increased stiffness is caused from the phenomenon that as a piece of steel is loaded elastically; it becomes increasingly hard to deform it. The preload eliminates the possibility of "easy" deformation; a greater amount of force is required.
There are a few different ways to add a preload:
- Pick a ball that is bigger than the groove
- Pick a nut that is smaller than the screw
- Pick a screw that is bigger than the nut
- Use 2 nuts and insert a wedge
- Use 2 nuts and insert a spring
- Offset grooves inside the nut
Ball Screw Assembly:
A ball screw must be supported so that the shaft can be rotated. Usually this involves roller bearings and the holding brackets. Some longer screws need to be supported for critical speed and bending restraints. End machining may be required to fit into the bearing or connect to the motor. Connecting to the motor requires a coupling and the appropriate machining. In order for the nut to translate accurately, it must be restrained by a table and support rails. In the end, a ball screw assembly should look something like the picture below.
What Type of Lubrication do Ball Screws use?
Ball Screws can use either grease or oil for lubrication. It is recommended to use grease for most normal applications. This is because grease does not require any special lubricating system and much less grease is needed than oil. Oil can be n