 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 