Torque, traction, and horsepower all play a role in propel motors. In this Shop Talk Blog post, we are going to review these subjects and how they are all related to each other in the context of final drive motors.
Here are some additional blog posts that you might find interesting ...
- Tracks vs Tires: Which has the Best Traction
- Introduction to Torque Density
- 5 Reasons a Final Drive Can Lack Power
Torque
Torque is a turning force applied to a shaft, and when it comes to travel motors it is what enables your machine to move. This is true whether we are talking about tracks or wheels. You need a certain amount of torque to be able to propel your machine and any load it may be carrying.
For a hydraulic motor, there are three types of torque: starting, stalling, and running torque. Starting torque refers to how much turning force can be generated by the hydraulic motor from a dead stop. Stalling torque is how much torque it takes to stop the motor. Starting torque and stalling torque for hydraulic motors are usually about the same, with starting torque around 75% to 80% of the maximum design torque of the motor. Running torque refers to how much torque is generated when the hydraulic motor is running.
Traction
That said, you can have a tremendous amount of torque at your disposal but it is useless if you can’t get any traction. Traction can be defined as the grip between the wheels (or tracks) on the ground. Traction is determined by the weight acting on the tires and by the coefficient of traction. I remember my mother's old 1985 Dodge Ram truck that had a tendency to fishtail until we put some bags of concrete in the rear of the bed to increase the weight over the back tires. This helped with the traction problem. The coefficient of traction is dependent on the two surfaces that are coming into contact: the tire (or track) and the ground. The graph gives you an idea of how the coefficient of traction varies for different surfaces, as well as how it differs between tires and tracks.
Note that tracks are often designed with grousers to increase traction, making up for the fact that tracks distribute the weight of the machine over the tracks. Tires, too, may have specially designed treads to increase traction between the tires and the ground.
Torque and Traction
Now, here’s where things can get interesting. In a situation where traction is very low (e.g., skid steer tires on ice), the torque will not exceed the traction. You can have the most powerful hydraulic motor on the market producing a huge amount of torque, but if the surface you are on is too slick then that torque won’t help you. That's why it's important to use the correct kind of tires or tracks for the surface you will be working on.
Torque and Speed
Remember that torque, speed, and horsepower are related:
(horsepower x 63025) / RPM = torque in inch-pounds
For a given horsepower, more speed means less torque. This inverse relationship between torque and speed is why most propel motors are low-speed, high-torque (LSHT) motors. That’s also why many hydraulic travel motors have a speed-reducing gearbox. By reducing speed, we multiply torque.
The figure below shows torque versus horsepower for specific speeds. You’ll notice that torque is higher for the same horsepower but lower speed.
Conclusion
When you need a hydraulic motor to move your machine (whether its a wheeled or tracked machine), torque, traction, and horsepower all play an important role. As long as your hydraulic motor is working properly and you've got enough traction, you should be in good shape.
Texas Final Drive is your partner in providing new or remanufactured final drive hydraulic motors from a single mini-excavator to a fleet of heavy equipment. Call today so we can find the right final drive or hydraulic component for you, or check out our online store to find your O.E.M. manufacturer brand motor now.
