What is a Ball Screw?
A ball screw, like all screws, is a rod with a helical groove that translates rotational motion into linear motion.
Why use a Ball Screw?
Common screws slide on their threads and pull the connecting piece with them. This is how an ACME screw works. This sliding motion is easy to produce but extremely ineffiecient; around 40% of the input power is transferred to the work piece. To improve on this idea the sliding contact must be replaced with rolling elements. Ball Screws use ball bearings in the nuts to roll along the screw. The balls must recirculate or they will fall out of the nut. This leads to the current designs for recirculating ball screws that have efficientcies greater than 90%.
Anaheim Automation's Ball Screw Offering
Anaheim Automation offers a high-quality line of ball screws offering flexibility and high performance. As a distributor of and TBI Motion ball screws, Anaheim Automation is sure to have the right ball screw for your application. Choose from a wide range of stroke lengths, as well as multiple different preload and accuracy ratings.
TBI Ball Screws: TBI provides Anaheim Automation with a low cost ball screw assembly solution that saves up to 50% next to competitors, without compromising quality. Both ground and precision rolled ball screws are available, from 4mm to 80mm in diameter. High precision grades from C0 to C10 are available, delivering up to 7,240kgf of dynamic linear force. Anaheim Automation will cut any ball screw to fit the length requirements of your application.
Physical properties of a Ball Screw:
The four main components of a ball screw include the ball screw shaft, ball nut, ball bearings, and the seal, further explained below.
1. Ball Screw Shaft:
The Screw Shaft is a long bar of chromium steel that has grooves formed into a helical pattern. This is the main body of a Ball Screw.
- Ball Groove design: When forming the groove for a ball screw it is important to think of the points of contact. The most common groove style is the Gothic Arch. This type of groove allows for four points of contact at all times. With four contact points the ball can be loaded in any direction in that plane.
- Pitch circle diameter of balls: The pitch circle (dm) is measured from the center of the bearing to the center of the bearing on the other side of the screw. This measurement is used for the maximum speed calculations.
- Nominal Diameter: The outside diameter of the screw. This measurement is useful for sizing the screw as it is easy to measure.
- Root Diameter: Diameter from the bottom of the groove to the bottom of the adjacent groove. This is the smallest part of the screw and is important for the critical speed calculation.
- Lead: It is the distance the nut travels in one rotation. The larger the lead the faster the nut can travel. The lead also determines the vertical maximum payload and the stopping power of the screw. This is because the load angle is steeper for greater leads; the amount of weight it can hold is less.
- Ball diameter: The diameter of the ball bearing that would fit into the groove with a preload of 0. This is used to calculate the diameter of the ball bearing that would give a certain preload. The bigger the balls compared to the nut and screw gap, the higher the preload.
2. Ball Nut (Nut and Ball Recirculation):
There are 3 main styles of Ball Screw Nuts each with a specialty:
- Profile Ball Groove: This is the most common type of ball nut, it is easy to make and has good consistent performance. Its simplicity makes this nut easy to mass produce and adjust to fit most sizes, leads, and loads. The balls are recirculated by tubes that pick up the bearings then deposit them back into the cycle. The number of rows of balls before they are recirculated is usually between 1.5-3.5 rows to keep the return tube size manageable and re-balling easier.
- Ball Pitch Circle Diameter: This nut has a trimmer profile because it does not use tubes to move the balls. Instead a deflector is used to bump the bearings into the previous track. This means that there must be a deflector for every turn, only one pitch circle per deflector. This design is also inherently better for smaller lead angles because the distance between the grooves is smaller and therefore easier to “jump.” This type of nut is more expensive to build because more of the track is internalized and its application is limited by the lead. Great for use on small, space-limited applications with small leads. Because of the recirculation method there can be only one row per cycle.
- End Cap: These types of nut are specialized for use with high lead and multiple start threads. It is reserved for these types of nuts because die molds are required to produce them, increasing cost. The number of rows per cycle is not limited in this recirculation type and it is normal to have the whole nut recirculated with one cycle per start.
Multiple Starting Threads:
A multiple thread screw is used in high lead applications to allow for more balls to be in contact with the nut. Multiple threads are only used on high lead screws because there is physically no room to add them on small lead screws. A good way to visualize a multiple thread screw is to think of a candy cane with multiple colors. The different colors are the different starts. End caps are used for these ball screws because they utilize less space to recirculate. Another thing to consider with multiple start screws is that the lead and the pitch are no longer the same. The lead is the pitch times the number of starts. See below diagrams.
3. Ball Bearings:
- Diameter: The bearing diameter must fit the lead and the shaft diameter. If the ball is too small there is too much space between threads; oppositely, if they are too big they are too close. The crest between the grooves should be flat, but not too big.
- Loading: There is no retainer between the balls like in circular bearings so the balls can become jammed when undesirable ball-on-ball contact is made. This can be reduced by adding spacer balls. Spacer balls are slightly smaller balls that spin between the load 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 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 oil (which requires a circulation system).
Required Maintenance for a Ball Screw
Backlash- the “lost motion” that comes from looseness in the drive train.
Ball Bearing- a chromium steel ball set to tight tolerances that support loads.
Ball Nut- the nut that rides a ball screw and uses recirculating ball bearings.
Ball Screw Shaft- a chromium steel shaft with helical grooves that house ball bearings to change rotational motion into linear motion.
Elastic Deformation- a changing in shape that is not permanent and will reverse on its own.
End Cap Nut- a style of nut that recirculates at the end, it is used to accommodate large and multiple start leads.
Grease- a lubricant that is not very viscous.
Lead- the linear distance traveled in one rotation.
Lead Error- the difference in the actual travel and the theoretical travel of the ball nut.
Lubricant- a substance that reduces friction between other objects.
Mounting Error- error caused from inaccuracies in mounting the screw in bearings, motors, ball nuts, etc.
Multiple Starting Threads- a screw that has more than one track, only applicable on large leads due to space restraints. Used to add more balls to large lead ball nuts.
Nominal Diameter- the diameter of the thread “tops;” the maximum diameter of the screw
Oil- a lubricant that is highly viscous
Pitch Circle Nut- a style of nut that recirculates the balls every rotation, good for small leads and small nut profiles
Pitch Diameter of Balls- the diameter created by connecting the centers of the ball bearings
Preload- the tightness of the nut to the shaft, caused from the elastic deformation of the parts involved.
Profile Ball Nut- the most common nut type, it uses tubes to recirculate the balls.
Root Diameter- the diameter made from the bottom of the screw, the thinnest point and used in life calculations
Seals- a device used to keep things in, out or both.