Helical Gear Rack for Linear Motion

Posted on Sep 26, 2020

Helical Gear Rack for Linear Motion

Helical gear rack and pinion assemblies are used to transform rotational motion into linear motion. The rack has straight teeth cut into one face of a square or round bar with a pinion, which is a small cylindrical rack that is meshed with the rack.

There are many kinds of sprockets in stock. If the application requires a long length that requires many racks connected in series, we have racks with correctly configured tooth edges. These are referred to as "machined end gear racks".

There are applications where the helical gear rack is stationary while the rack is moving and others where the rack rotates around a fixed axis while the rack is moving. The former is widely used in transport systems, while the latter can be used in extrusion systems and lift / lower applications.

As a mechanical element to transfer rotary into linear motion, gear racks are often compared to ball screws. There are pros and cons. The advantages of a gear rack is its mechanical simplicity, large load carrying capacity, no limit to the length, etc. One disadvantage though is the backlash. The advantage of a ball screw is the high precision and lower backlash while the shortcomings include the limit in length due to deflection.


Rack Types

Rack and pinion gears are available in two variations:

Straight teeth have the tooth axis parallel to the axis of rotation. Straight teeth that run parallel to the axis of the gear. Helical teeth gears provide continuous engagement along the tooth length and are often quieter and more efficient than straight tooth gears and also offer higher loading for same rack width. Helical tooth gears resemble spur gears in the plane of rotation, but include teeth that are twisted along a helical path in the axial direction.


:: Read more : Helical gears or helical rack and pinion?


Helical gear rack drives are ideal for a wide range of applications, including axis drives requiring precise positioning and repeatability, sliding gates and columns, pick & place robots, CNC routers, and material handling systems. These drives can also easily handle heavy loads and duty cycles. Industries served include material handling, automation, automotive, aerospace, machine tools, and robotics.

Linear motion is necessary for the movement of machines; transports tools and products in an efficient and controlled manner. Linear motion generators are generally ranked according to their axial velocity and acceleration, axial forces against structure volume, durability, stiffness, and positioning accuracy.

Two popular linear systems are linear motors and screw drives. Rack and pinion drives are often overlooked as previous generation technology with limited positioning accuracy. However, this assumption is invalid.

Precision-ground mounting surfaces with tight tolerances, wear-resistant surface treatment, individually deburred gear teeth and light-weight compact designs increase productivity. In fact, rack and pinion drives compare favorably with linear motors as well as ball screws with a shaft or earth thread.

The new generation rack and pinion systems offer high dynamics and unlimited travel distances. Some of these include premium quality servo gears and actuators with less than 1 arcmin of clearance, up to 98.5% efficiency, and much more compact dimensions than standard servo and gear combinations. Some pre-assembled gears can work with accuracy down to 10 µm, which ensures safety and smooth movement.

Typical rack and pinion applications include gantry, transport, and packaging machines ranging from a few pounds to several tons. The new generation of rack sets are also used in woodworking machines, high-speed metal cutting machines and assembly machines.


:: Read more : Helical gears or helical rack and pinion?

Geometry and Surface Details

The performance of the rack has improved with the overall technological progress. For example, state-of-the-art machining and grinding have greatly increased the precision of the rack and pinion mechanism.

More specifically, some high-quality stand components are laser etched for a cumulative pitch error of ± 12 µm over a length of 500 mm, allowing manual selection of target accuracy. This is useful for parallel alignment of racking components in dual drive gantry applications. In fact, this level of precision allows several types of machines to run without external feedback devices; and other linear systems require expensive external feedback devices for commutation and positioning.

Helical rack with optimized helix angle is preferred for quieter operation at higher speeds and higher load capacities due to the higher tooth contact ratio. The error of one pitch between the helical teeth can reach 3 µm. A pinion profile shift or addendum modification prevents undercut; it also balances bending stresses, for higher load capacity. Helical gearing engages smoothly and quietly — which helps improve surface finish, for example, when machining tight-tolerance parts.


Rack-and-Pinion Integration

There are many options for mounting rack kits. Some racks use special mounting surfaces to ensure accuracy, while others provide adequate performance even with basic installation. The natural flexibility of the design can be used for better control: unlike direct drive linear motors, rack sets allow you to adjust pinion size, gears and damping - to stabilize closed-loop control.

There are pitfalls: too much spacing of the pinion and rack causes play which lowers precision. A damaged or misaligned mount can also damage the gearbox bearings - causing more motor current draw, noise and even failure. For best performance, the pinion should be sufficiently spaced from the rack, mounted flat, and perpendicular to the gear to an accuracy of about 25 µm for many applications.

Advances in rack and pinion gears and the decline in servo prices mean that servo motors are typically coupled to rack systems. Stepper motors are a viable option, but servomotors are preferred for their precision.


:: Read more : Everything you need to know about Rack and Pinion Gears


Sometimes sprocket sets are preloaded to eliminate play and increase stiffness. Here, the two gears move on the same rack. The main pinion drives the mechanism as in the normal configuration; Meanwhile, the auxiliary pinion gear can generate torque to apply counter force to the teeth it is in contact with. Thus, the inertia and resistance prevent backlash, even during load changes; system stiffness also increases and improves steering dynamics.

If the components are selected correctly, there are no significant shortcomings in preloading the rack system. On the other hand, mechanical preload can actually reduce the overall stiffness of the machine. For example, a split spring loaded pinion would reduce the stiffness of the system:

Note that, unlike more sophisticated electronic preload systems, these traditional preload pinions cannot work together; one always goes against the other, which slightly reduces efficiency.

In more sophisticated rack sets, the electronic preload is kept to the maximum while the system is stationary. Main and auxiliary gears - both actively powered - press in the teeth of the racks facing in opposite directions. Then, as the machine accelerates, the primary pinion drives the machine forward while the secondary pinion reduces the preload of the opposite force. When the system decelerates to a constant speed, the auxiliary gear comes into contact with the side of the tooth that corresponds to that which is coupled to the main pinion; then the two gears move in the same direction while preventing backlash.

Finally, as the system slows down, the auxiliary pinion reverts to applying force to the opposite side of the tooth to help slow the load.


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