e 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 