Ball & Acme Screw Products
The main function of ball and acme (lead) screw assemblies is to provide a drive mechanism, usually within a positioning table, to move a load. The drive mechanism is an element that contributes to position accuracy, repeatability, speed, and mechanical system resolution.
What are Ball Screws?
Ball Screws are the screw of choice for high duty cycle, high speed, and long life applications. The ball screw nut uses one or more circuits of recirculating steel balls which roll between the nut and ball screw grooves, providing an efficient low friction mechanical drive system. The nut, which is attached to the table carriage, moves back and forth in parallel with the linear bearing system as the ball screw rotates. Using a higher lead ball screw (for example a 0.500 inch lead instead of a 0.200 inch lead) will offer greater carriage speed for applications requiring rapid traverse, or fast, short incremental moves. Key features of a ball screw system are low wear and long life.
- Rolled Ball Screw system utilizes a tapped nut with a standard accuracy grade rolled screw.
- Precision Rolled Ball Screw system utilizes a ground nut with a higher accuracy grade rolled screw.
- Ground Ball Screw system utilizes a ground nut with a high accuracy precision ground screw, and provides the highest performance and accuracy.
What are Acme Screws?
Accuracy grades of acme screws and ball screws are typically reflected by their "lead error" specification, which is generally given in a "in/ft" (microns/300 mm) rating. This "lead error" is the primary element in determining the position accuracy of a positioning table. The lower the "lead error" (better position accuracy), the more costly the drive screw assembly.
Acme Screws typically use a turcite (polymer) or bronze nut. The nut, which is attached to the table carriage, moves back and forth in parallel with the linear bearing system as the acme screw rotates. The threads of the plastic or bronze nut ride in the matching acme screw threads, much like an ordinary nut and bolt system. This produces a higher friction (thus lower efficiency) system than a ball screw assembly, since there are no rolling elements between the nut and the acme screw threads. Acme screws work fine for applications requiring low speeds, low noise and low duty cycles. Acme screws are also a good choice for most vertical applications as they typically prevent back-driving of the attached load. However, in some applications the friction of the acme screw can affect low speed smoothness and its long term life is significantly lower than a ball screw.
A turcite nut option is available with rolled ball screws and operates very similar to an acme screw. However, the polymer nut contacts the ball screw grooves differently than an acme screw assembly producing a drive mechanism that has less friction than an acme screw thereby allowing for potential higher speeds and less input torque required from a coupled motor. This option provides smooth motion, low audible noise, and is ideal for vertical applications. Also, using the turcite nut with high lead ball screws provides for faster linear speeds.
Technical Considerations View More
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|
|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.
|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.
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.