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 needed for high-speed applications or when required.
History of Ball Screws:
There have been attempts to replace sliding friction screws with rolling friction screws since the 19 century. However because of the limited level of manufacturing and technology, the first practical application of ball screws was by General Motors, who used ball screws for their steering columns. Since then the technology surrounding ball screws has advanced rapidly. This rapid advance has led them to be used in CNC machines, automated manufacturing, and medical devices. With superior accuracy, repeatability, and high efficiencies ball screws are the best way to translate rotational motion into linear motion.
Ball Screw Applications
Ball screws are used in motion control but require bearing supports and motor couplings to work effectively. By using ball screws in conjunction with linear stages we are able to produce stable linear translation for many applications. For this reason ball screws are used in a broad range of industries ranging from garage door openers to CNC machines and MRI table sliders. Ball screws are not commonly used in applications requiring oscillating motions less than a ball diameter. This type of motion removes the lubrication layer and leads to increased error. This can be worked around using smaller ball screws and special grease.
Lifetime of a Ball Screws
Ball screws have very low friction coefficients, which is the key to both their accuracy and their longevity. The actual life is determined by load, speed, lead and screw size. Common values are 20,000 hours, and 50,000 km or 5 years.
Required Maintenance for a Ball Screw
Most ball screws require a certain degree of regular maintenance; however some specialty balls screws do not require any maintenance! Make sure that the rails are free of contaminates like water, dirt, dust and debris. The nut should also be lubricated using either oil or grease. Grease offers lower overall performance but is much more convenient, cheaper to use, and easier to maintain than o