A very controversial topic, there is plenty of information (not always true) out there about both ball screw and rack and pinion drive systems. Both are used across many industries and transfer rotational motion into linear motion systems to move and position machines.
With a team of over 30 in-house engineers and 65+ years building precision machinery, Park Industries® is well-versed in this debate. Park uses both systems while designing durable machinery for the stone and metals industries. Below, we take a look at each system, and highlight a few pro’s / con’s of them.
Rack and Pinion Drive Systems
What is it?
A rack and pinion drive system is just what it sounds like. There is a rack that is bolted onto the sides of the machines and pinion, or cog/gear, that meshes with the rack. As the pinion turns, the machine moves. Straight racks and helical (angled) racks merge to drive.
What does it do well?
Best known for their rigidity, rack and pinion drives can deliver precise motion over unlimited lengths. They are built to travel the full distance of a machine, no matter the length, without “screw whip” concerns (more on that later). Rack and pinion systems have high up-time and can produce for years with minimal wear and are very favorable in the environment of a fab shop, being virtually impervious to dust and debris.
Helical rack and pinion designs are angled instead of straight, with more teeth in the mesh of the rack. The helical design creates a smooth, low-friction movement and delivers zero backlash with a positional accuracy of ±.001″ throughout the entire travel distance of the machine. They also excel in accelerating, de-accelerating, and maintaining higher speed processing which is a result of their constant stiffness and exceptional transfer of power, or efficiencies, that are upwards of 97%.
The simplicity in the design of rack and pinion is a huge plus, benefiting end users. They are usually simple bolt-on systems that are easy to replace – not requiring special skills or knowledge.
What are its weaknesses?
Old rack and pinion technology was known for their cons of having higher friction and potential backlash of the pinion. Advances in gear production techniques (milling, grinding, and heat treatment, for example) have dramatically improved the precision and load carrying capacity of rack and pinion drives, to the point that they are a competitive component for any linear-axis drive application.
|Rigidity||Potential backlash on older models|
|Accuracy not restricted by length||More complicated to manufacture|
|Easy to service||Less mounting options|
|Better equipped for faster speeds|
|Extremely efficient transfer of power|
|Extremely resilient to contamination in harsh environments|
Ball Screw Drive Systems
What is it?
Made up of a nut, screw, and ball bearings, ball screw drive systems work like typical power screws but the rolling friction replaces the sliding friction. As the screw rotates, the machine moves.
What does it do well?
When done correctly, ball screws have exceptional accuracy and are friction-free, sliding smoothly with the motion. There are also more mounting options. Racks have to be bolted to something for their entire length, where ball screw may be a better fit for some machine designs. Ball screws systems are also easier to use in the manufacturing process, where rack and pinion may require more processes to be built correctly.
What are its weaknesses?
As stated above ball screws are known for being accurate and friction-free. However, on a longer axis, vibration can worsen as increased speed is needed, hence making the screw rotate faster. This issues is coined as “screw whip”. A ball screw driven system may need bigger motors or more gearing compensation for “screw whip” and to maintain machine positioning speeds, making the use of a ball screw on axis over 4’ in length not ideal. Ball screw systems are best suited for height positioning or the Z axis because of the short travel distance.
In regard to speed, ball screws cannot accelerate as quickly nor can they maintain the higher speeds as efficiently. Their stiffness is lower and less constant. Their transfer of power, or efficiencies, is in the 80-85% range.
Serviceability is also trickier with ball screw systems, usually requiring specialized skills to replace which isn’t ideal being that ball screws can be very sensitive. Ball nuts are very sensitive to misalignment, especially when opposing nuts are used. Ball screws are also more susceptible and less forgiving to contamination and harsh environments (like stone shops) which can lead to catastrophic failures.
|Less friction||Crashes can damage ball screw|
|Accuracy||More expensive with less availability|
|Easier manufacturing||Performance declines on longer axis (Screw Whip)|
|More mounting options||Hard to get to for service/replacement|
|Less efficient in transfer of power|
|More prone to contamination and failure in harsh environments|
What Does Park Industries® Use?
Park Industries® has found a mix of the two drive system as the best design for machine durability and machine up-time. Park uses the latest helical rack and pinion drive systems for most applications including length and width travel (axis Y and X). For smaller motion applications (Z axis / height positioning), a ball screw system is used.
We find the rack and pinion systems to be extremely efficient, durable, and easy to service. For example, of the over 700 active TITAN® CNC Routers in production throughout North America, less than 1% of them have needed replacements for their rack and pinion systems.
In addition, rack and pinion systems allow for faster movements, optimized for running high speed tooling to increase your machine efficiency. Due to their rigid build and quality, Park Industries® CNC machinery has always been able to run tools and cut at higher speeds. Advancements in the tooling industry are finally catching up to what our machines can really do, allowing operators to gain speed without sacrificing quality.