Rack and Pinion Backlash

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The rack and pinion components of a machine are a pair of gears that convert rotational motion into linear motion. The rack and pinion compose a type of drive system. As discussed in the Ball Screw vs Lead Screw article a drive system is instrumental in driving the machine along its axis. Machines can come equipped with either style of drive system, depending on what model, style, and what is being cut. These components can have significant effects on cutting speed, accuracy, and energy use, so understanding them is essential in order to obtain optimal performance from your machine. One of the most important concepts that can have the greatest implications on performance in relation to the rack and pinion is called rack and pinion backlash. The following article will explore this aspect of the rack and pinion drive system to give you a greater understanding of how it can affect your machine. Lets first take a look at the two components, the rack and pinion:

Pinion

The pinion is the circular gear that moves in a rotating fashion. It has teeth that interlock with ones on the rack and so when the pinion turns it cause the rack to move in a linear fashion. The rotational motion is therefore converted to linear motion.

Rack

The rack is the straight linear piece of the gear system that harnesses the rotational energy from the pinion toward its own linear motion. It is a long and straight component that has teeth matching those on the pinion.

Rack and Pinion Backlash

As mentioned above, the rack and pinion are components that each have ridges or teeth on them. These teeth fit together and allow for the rotary motion of the pinion to be converted into the linear motion of the racks. When the teeth fit together there is a small space between the tooth of the pinion and the groove between teeth on the rack. The small space causes some of the rotational energy to be lost in conversion. This is caused because the space between the two would create a loss in motion when the movement of the gear is reversed. Since the pinion tooth in contact with the rack does not touch both sides of the groove, when motion is reversed, the tooth moves for a brief moment without having any affect on the rack. Therefore that amount of rotary energy is lost and will not be harnessed towards driving the movement of the machine. One way to decrease rack and pinion backlash is to be attentive to the teeth quality and arrangement. There are two main styles for the teeth on both the rack and pinion:

  • Straight: The teeth are straight across on the rack and pinion.
  • Helical: This style has the teeth at an angle, which allows them to run more quietly as well as have a greater load capacity. Since angling the teeth removes the constraint of the width of the rack or pinion, they can have a greater length. This allows for the teeth to be in contact for longer, creating the higher load capacity.

Although rack and pinion backlash appears to have negative implications for machine performance, it is also important not to eliminate it. While you do not want too much backlash, it is almost impossible to have a system operating with zero backlash. If there is no space between the pinion tooth and the two teeth on either side then there is a significant chance that the gears will end up locking. Zero backlash can also decrease the life of the parts, as it leads to greater wear and grinding.

However there is a way to achieve zero backlash and prevent the negative effects of it. The way to obtain these conditions is through using what is called a dual pinion or split pinion system. This means that there are two pinions instead of one. While one of the pinions drives the rack, the other one works to eliminate the backlash. The elimination of backlash allows for the conversion of motion to be optimized while at the same time preventing binding, grinding, and excessive wear of parts.

 

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