Helical rack and linear motion are necessary for moving machines; transports tools and products in an efficient and controlled manner. Linear motion mechanisms are generally ranked by velocity and axial acceleration, axial forces as a function of construction volume, durability, rigidity and positioning accuracy.
Two popular linear systems are linear motors and screw drives. Rack drives are often overlooked as the previous generation technology with limited positioning accuracy. However, this assumption is incorrect.
Precision-ground mounting surfaces with narrow tolerances, wear-resistant surface finishes, individually removed gear teeth, and compact, lightweight designs increase performance. In fact, rack and pinion drives are favorable compared to linear motors as well as ball or roller screws.
New generation gear systems offer high dynamics and unlimited travel distance. Some include top-quality servo drives and actuators with less than 1 arc clearance, up to 98.5% efficiency, and much more compact sizes than standard servo-gear combinations. Some pre-assembled gears can work even with an accuracy of 10 µm, which ensures safety and smooth movement.
Typical applications for gears include gantry, transport and packaging machines that transport from a few pounds to several tons. New generation gear sets are also used in woodworking, high-speed metal cutting machines and assembly machines.
The rack performance has improved with overall technological advancement. For example, state-of-the-art machining and grinding have a much higher gear precision.
More precisely, some elements of the premium rack are laser etched for a cumulative stroke error of ± 12 µm over a length of 500 mm, which allows manual selection of target accuracy. It is useful for parallel matching rack elements to double-gate gates. In fact, this level of precision allows several types of machines to work without external feedback devices; on the contrary, other line systems require expensive external commutation and positioning devices.
A helical rack gear with an optimized helix angle is preferred for quieter operation at higher speeds and higher load capacities due to a higher contact ratio. A single pitch error between helical teeth can reach 3 µm. Changing the pinion profile or modifying the attachment prevents undercutting; it also balances bending stress, providing higher load capacity. The helical gearing interlocks smoothly and quietly - which helps to improve the surface finish, for example when machining parts with low tolerances.
Lubrication is key
The gear sets are the longest if they are properly lubricated. Properly lubricated sets are also the most capable of achieving the highest rated speed. In many rack systems, the most common method is an automatic lubrication kit or lubrication device.
The expensive external switching devices for commutation and positioning. These devices come in various sizes or volumes and are controlled electronically.
You can select different settings to control the amount of grease that flows over time - depending on the gear cycle of the rack and pinion. A charged container maintains pressure when not in use; closing the two-wire switch activates the flow.
The grease travels through the hose to the hollow lubrication pinion, felt gear with radial holes, in which grease is applied to the rack or pinion through the holes. Here, the design determines which half of the assembly is actively lubricated: For example, lubrication of a high-speed application rack can prevent grease recoil.
In any case, the correct amount of grease required for the application can be applied automatically and accurately, for little maintenance.
Mounting options abound in gear sets. Some cabinets use special mounting surfaces to ensure accuracy, while others provide adequate performance even for basic installation. For better control, you can take advantage of the inherent flexibility of the design: Unlike direct-drive linear motors, rack kits allow you to adjust pinion size, gear ratios, and damping - to stabilize closed-loop control.
There are pitfalls: too much spacing of the teeth of the gear and rack causes slack, which reduces precision. A damaged or misaligned assembly can also damage the gearbox bearings - resulting in greater engine power consumption, noise and even failure. For best performance, the pinion should be properly spaced from the rack, mounted on a flat surface, and perpendicular to the gearbox up to about 25 µm for many applications.
Advances in gearing and falling servo drive prices mean that usually servomotors are paired with rack systems. Stepper motors are a viable option, but servo motors are preferred because of their precision.
Sometimes rack sets are preloaded to eliminate backlash and increase stiffness. Here, two gears operate on the same rack. The main pinion drives the mechanism as in the usual configuration; meanwhile, the secondary pinion can generate torque to apply the force opposite to the teeth it engages. In this way, inertia and resistance prevent slack, even during load changes; system rigidity also increases and increases control dynamics.
If the components are selected correctly, there are no significant disadvantages in the preloading of the rack and pinion system. On the other hand, mechanical preload can actually reduce the overall rigidity of the machine. For example, a spring split mechanism would reduce the rigidity of the system:
Note that, unlike the more sophisticated electronic pre-charging, these traditional pinions cannot work together; one always opposes the other, which slightly reduces performance.
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