What’s Better, Rack-And-Pinion Or Ball Screw?

The rack-and-pinion transmission system refers to a rack bolted to the side of the machine and pinion gear or gear/gear that meshes with the rack. As the pinion rotates, the machine moves. In the CNC router machine industry, spiral (angled) frames are widely used to incorporate drives.

Advantages of rack and pinion transmission system

The rack and pinion drive is known for its rigidity and can provide precise motion for an unlimited length of time. They are constructed to travel the entire distance of the machine regardless of length, without worrying about “spiral whiplash” (more on this later). The rack and pinion system has a long normal operation time, can be produced for several years, with minimal wear, and is very beneficial in the environment of the factory floor, almost free from dust and debris.

The design of the helical rack and pinion is angled, not straight, and there are more teeth in the mesh of the rack. The spiral design enables smooth, low-friction movement and provides zero backlashes with a position accuracy of ±.001 inches throughout the machine stroke. They also have excellent acceleration, deceleration, and processing capabilities to maintain higher speeds, which are the result of their constant stiffness and excellent power or efficiency transfer (up to 97% or more).

The simplicity of the rack and pinion design is a huge advantage that benefits the end-user. They are usually simple bolted connection systems that are easy to replace-no special skills or knowledge are required.

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Disadvantages of rack and pinion transmission system

The processing accuracy and processing technology requirements are high. If processing technology and processing technology cannot be achieved, there will be high friction and potential gaps between the rack and pinion. Fortunately, with the advancement of production technology, such as the improvement of milling, grinding and heat treatment processes, the transmission accuracy and load-carrying capacity of racks and pinions have been greatly improved.

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Ball screw is an ideal product that converts rotary motion into linear motion or converts linear motion into rotary motion. The ball screw is composed of a screw, a nut, and a ball. Its function is to convert rotary motion into linear motion, which is a further extension and development of the trapezoidal screw. The important significance of this development is to change the sliding of the screw into rolling and reduce friction. Because of its small frictional resistance, ball screws are widely used in various industrial equipment and precision instruments.

Advantages of ball screw

After the operation is completed correctly, the ball screw will have excellent accuracy, will not produce friction, and can slide smoothly. There are more installation options. The frame must be bolted along the entire length, and ball screws may be more suitable for certain machine designs. The ball screw system is also easier to use in the manufacturing process, in which case the rack and pinion may require more processes to build correctly.

Disadvantages of ball screw

As mentioned above, ball screws are known for being precise and frictionless. However, on longer shafts, the vibration may get worse as the speed needs to be increased, thus making the screw spin faster. This problem is called “screw whip”. A ball screw-driven system may require a larger motor or more gear compensation to compensate for the “screw” and maintain the positioning speed of the machine, so it is not ideal to use a ball screw on a shaft longer than 4′. Due to the short-stroke distance, the ball screw system is most suitable for height positioning or Z-axis.

In terms of speed, the ball screw cannot accelerate at the fastest speed, nor can it maintain a higher speed with higher efficiency. Their stiffness is lower and less constant. Their power or efficiency transfer is in the range of 80-85%.

The maintainability of the ball screw system is also tricky, and usually requires specialized skills to replace it, because the ball screw can be very sensitive, so this is not an ideal choice. Ball nuts are very sensitive to misalignment, especially when opposing nuts are used. Ball screws are also more susceptible to contamination, and will not tolerate harsh environments that can lead to catastrophic failures (such as stone shops).

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• Carrying capacity

The load-bearing capacity of the rack and pinion is stronger than that of the ball screw and can work under high loads and heavy loads. The rack and pinion can carry a maximum load of 15-20 tons.

• Positioning accuracy

The positioning accuracy of the rack relative to the ball screw is not so high.
The rack and pinion design may compromise the overall accuracy of the machine. Factory-produced gears cannot produce the resolution or tolerances of ball bearings. This is why when you look at any industrial machinery, you will find that the most critical axis (Z-axis) has a ball screw.

Recirculating ball bearings can be used in all linear guide systems and are trusted technology from high-end CNC router machines to high-end aerospace and metalworking router centers. Your CNC router should have the same technology so that you can work with the most advanced and accurate system.

• Moving speed

The moving speed of the rack and pinion is not as fast as the ball screw.

Ball screws can move quickly, but in simple terms, rack and pinion systems can usually provide greater linear speeds, usually with large amplitudes. If you want to shorten the cycle time, the limitations of the ball screw will only get worse as the length increases.

• Installation requirements

There are requirements for the rack and pinion installation, clearance must be reserved, and reverse gear gauges are required for installation.

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• Wear condition

Rack wear needs to be compensated, and ball screw wear needs to be adjusted.

The ball screw system improves the level of wear. The ball screw on the table of our CNC router machine consists of a sealed bearing and many recirculating ball bearings, which are evenly distributed and continuously lubricated when passing through the channel design.

Rack and pinion do not have this technology, but the gear meshes with the track without lubrication (metal-to-metal contact). You must manually apply grease to the track, otherwise, you will be contaminated by dust, dirt, and other debris when cutting. This reduces lubrication and increases the wear of the gears and their tracks. As time goes by, the gear/rack starts to wear and the tooth pitch further increases (increased clearance), which ultimately changes the overall accuracy of the table. This is caused by the rack and pinion system having to stop at the same point (teeth) over and over again. Our ball screw design does not have this problem, because many of the ball bearings of the ball screw have an unlimited number of contact surfaces that can be parked on.

• Manufacturing difficulty

The rack is easier to manufacture than the ball screw.

• Price

Rack-and-pinion is cheaper than ball screws. Generally, it is economical to choose a rack and screw for precision.

As the length increases and the requirements for force increase, precision ball screws become more and more expensive, especially when custom end machining or modification of the nut is required, or when the size needs to be increased to reduce tension or buckling force.

On the other hand, the rack and pinion can provide a high level of positioning accuracy at a relatively reasonable cost. The complexity of manufacturing components is much lower, and longer performance can be more reliable.

• Delivery time

Since almost every ball screw application requires a custom length and end finishing, the lead time can be very long. It is not uncommon to spend weeks to tens of weeks to obtain a customized ball screw assembly.

On the other hand, rack and pinion systems can usually be delivered from stock components, and only the last piece of the rack in the system needs to be simply cut.

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Both the precision ball screw system and the rack and pinion system excel in their own specific ways.

Ball screws are known for being very smooth and almost frictionless. For short-distance accuracy, ball screws are very suitable. This is why we let it handle the Z-axis. Their disadvantages include the need for precise alignment, contamination sensitivity, and the need to increase the diameter and weight related to length (span) to prevent whipping or vibration. In turn, this requires more gear units or larger motors to compensate for the weight and needs to maintain a fast positioning speed.

Rack and pinion drives are known for their rigidity and are not limited by length. The continuous stability of the precision ground helical gear system enables our CNC to run for a long time. When properly installed, they can be used for a long time with minimal wear and tear, and they are hardly affected by dust and debris. The disadvantages of rack gears include low efficiency (due to higher friction), backlash caused by poor pinion meshing, and vibration caused by tooth shape.

In summary, with regard to rack and pinion VS ball screw, I believe you now know how to choose them. Generally speaking, for small CNC machine tools of 2X2, 2X3, 2X4 ft size, the XYZ axis can all be ball screws, and for large machine tools such as 4X8, 5X10 or larger size machine tools, the XY axis frame uses pinion gears, Z It is more reasonable to use a ball screw for the shaft. If you have any questions about this, you can consult us for free, and we will recommend the most suitable machine configuration for you.

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