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MOTION CONTROL AT THE SPEED OF TECHNOLOGY™
Ball & Acme Screw Technical Considerations
Preloading of an acme or ball screw nut is the process of eliminating backlash within the nut assembly. Eliminating the backlash in a nut becomes important in applications requiring good bidirectional positioning and repeatability. Also, eliminating nut backlash can help stabilize a motion controller operating in a closed loop system. Some drive systems use two nuts that are separated by a spring (or spacer) which provides a force between the two nuts. This process is used with acme screw, rolled ball screw, and precision ball screw assemblies. The ground ball screw option typically eliminates backlash by using "oversized" recirculating ball bearings in one nut that is matched to the screw thread.
Resolution of a screw driven positioning table is directly related to the lead of the screw. The lead of a screw (also known as the screw pitch) is the distance traveled by the nut (or carriage) for one revolution of the screw. Therefore, a screw with a 0.200 inch lead will travel 1.000 inch when the screw rotates five (5) revolutions. While a screw with a 0.500 inch lead will travel 1.000 inch in just two (2) screw revolutions. Using a screw with a 0.200 inch lead will provide better mechanical resolution over a screw with a 0.500 inch lead. However, the 0.500 inch lead screw will require more motor torque to move a given load.
For most acme screw driven positioning table applications, the screw (and not the linear bearing) is the major factor in determining the life of the positioning table. This is due to the high friction of the nut assembly. In contrast, for most ball screw driven positioning table applications the linear bearing system (and not the screw) is the major factor in determining the life of the positioning table. This is due to the high efficiency and high load capacity of the nut.
Because acme screw & turcite nut screw driven positioning tables have a solid surface contacting a solid surface, providing sufficient lubrication is a must. If there is insufficient lubrication, the high frictional forces of these nuts will cause excessive wear to the assembly, thus reducing positioning table life dramatically. Applying lubrication directly onto the entire length of the acme screw on a regular basis is highly recommended.
For ball screw driven positioning tables, the many built-in pockets within the ball nut assembly will allow the adhesive properties of most lubricants to be stored and used by the recirculating balls for extended periods of time. Thus, as the positioning table moves back & forth lubrication is applied to the ball screw as the ball nut assembly picks up grease or oil that is spread over the entire length of the screw. Therefore, applying lubrication directly onto the entire length of the screw on a regular basis is also recommended.
The chart below compares the primary differences between Ball and Acme Screw drive systems.
| Consideration | Acme Screw | Ball Screws | Comments | ||
|---|---|---|---|---|---|
| Rolled | Precision | Ground | |||
| Audible Noise | Least audible noise | Most audible noise | Less audible noise than rolled screw | Less audible noise than precision screw |
Acme: no rolling elements provide for quiet operation. Ball: recirculating balls in nut assembly transmit audible noise during motion; due to more accurate machining procedures - precision & ground ball screws are quiter than rolled ball screws. |
| Back Driving Loads | May prevent back driving | Can easily back drive a load | Can easily back drive a load | Can easily back drive a load |
Acme: good for light loads & vertical applications. Ball: recirculating balls in nut assembly producte a low friction system; vertical applications may require a brake to hold the load when no power is applied to the motor. |
| Backlash Non-preloaded nut |
Will increase with wear | Constant | Constant | Constant |
Acme: preloaded nut assembly eliminates backlash. Ball: preloaded nut assembly eliminates backlash. |
| Duty Cycle | Low to medium (< 50%) | High (100%) | High (100%) | High (100%) |
Acme: low duty cycle due to high sliding friction. Ball: high duty cycle due to recirculating balls in nut assemble - high efficiency & low friction system. |
| Life | Shorter due to higher friction | Long | Long | Long |
Acme: mechanical wear related to duty cycle, load & speed. Ball: minimal wear if operated in proper environment, within load specifications, and periodically lubricated. |
| Relative - Cost | Slightly more than rolled ball | Least expensive | Slightly more than rolled ball | Most expensive |
Acme: a little more expensive than the rolled ball screw. Ball: due to more accurate manufacturing procedures precision rolled & ground ball screws are more expensive. |
| Screw Efficiency | low (40% Acme, 60% Turcite) | High (90%) | High (90%) | High (90%) |
Acme: low efficiency due to high sliding friction. Ball: high efficiency sue to recirculating balls in nut assembly - low friction system. |
| Smoothness | Can be smooth | Least smooth | Medium smoothness | Smoothest |
Acme: due to friction can start/stop at very low speeds. Ball: smoothness is constant through a wide speed range; due to more accurate manufacturing procedures precision & ground ball screws are smoother than rolled ball screws. |
| Speeds | Low | High | High | High |
Acme: high friction can cause excess heat & wear at high speeds. Ball: recirculating balls in nut provide for a high speed system due to low friction & high efficiency. |
