Let’s consider the gear drive.
Gears allow the motor to be close to the leadscrew and this allows the motor to clear the base casting on the X-axis. It also lets the motor be pointed in towards the machine so the handles barely stick out farther than stock. Gears also increase resolution and torque.
Advantages of gears
- They are less expensive than two pulleys and a belt
- Gears place the motor and driven shaft close together at a fixed distance. Belts usually have a much larger pulley to pulley distance. Being able to change center distance without affecting ratio is sometimes an advantage of belts. In this application this is not useful
- Larger ratios are available with gears. A belt drive with a large ratio has a small pulley that must be spaced at a distance to get enough wrap around the pulley. A gear set can reduce speed considerably and still be tucked up close
- Belt drives need some sort of way to adjust belt tension. Tension places side loads on shafts and bearings that are far greater than driving the load. Lack of a tensioning feature on gear drives makes the assembly more simple and compact
- The tight package of the gear assembly allows it to be mounted with the motor below the table, above the table or to either side if desired
What is the error caused by play between gear teeth?
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Gear load?
The load on the gear is pretty low. The motors don’t have to generate much torque to move the table. Think how little effort it takes to crank the handles on even a large cut. Some CNC retrofits use motors with as much as 1600 oz-in of torque. This much torque has the potential to damage the machine if it runs out of travel or something interferes with a spoke of the handle or any similar event. Being able to overpower the motors by hand is an important feature if the cutter veers off in the wrong direction and you want to stop it quickly.
What about wear?
The design uses a combination of Delrin and aluminum. Plastic to metal is ideal for gearing. The theory says, during the break-in period, plastic is transferred to the metal and fills up the microscopic pits on the aluminum gear and afterwards the wear is extremely low. However in this application the PV value, that is, pressure x velocity is very low and it may not even be enough to initiate the bedding process which means there isn’t even initial wear. In engineering terms the life is infinite. There has never been a report of gears wearing out.
Gear noise
Using plastic against metal teeth drops noise considerably. In most cases the gear noise is hidden by the sound of the stepper motors ringing. My videos tend to amplify certain frequencies in the camera’s attempt to capture voices. Nonetheless, a belt drive is usually quieter than gears.
