OP has messaged me that his problem is understanding how the spring force setup measures torque. Let's see if this furnishes the answer he wants:
Imagine an electric motor which is connected to a load. Assume that it is performing work on the load, which means that
- the motor is exerting a force on the load
- the load is moving
Assuming a constant force, the net work done by the motor on the load is then w = (force) x (distance) and the power is p = (force x distance)/time.
Since velocity = distance/time, we can rearrange the power equation to read p = (force) x (velocity) and since we are talking about rotary motion, the force involved is a twisting action and the velocity is a rotational speed. Thus the power equation is translated to
shaft horsepower = (torque) x RPM, taking care to keep our units consistent.
Now we are ready to look at the motor and the apparatus connected to it. First we note that the electric motor is applying a twisting force (torque) to the load which means the load is applying an equal and opposite twisting force to the motor. this means the motor casing is applying that same twisting force to the foundation it is bolted to, which means those bolts are transferring the torque reaction. In turn, this means that if we can determine the stresses in those mounting bolts, we can solve for the torque being produced by the motor, multiply that torque by the RPM at which the motor is running, and then know the power being produced by the motor and absorbed by the compressor.
The diagram above shows the motor is mounted on a pivot so that anything that applies a torque to the motor will cause the entire motor to rotate about that pivot. To prevent this from happening, a spring force gauge is also connected to the motor mount through a lever. the spring force gauge is simply a spring which deflects a known distance under the influence of a force and hence lets you measure that force.
And here is the solution: the spring force gauge can be thought of as a mounting bolt which lets us measure the stress in it which is exerted upon it by the motor. the force that the spring gauge senses is being applied to the motor via a lever arm as specified in the drawing, and since torque = force x lever arm, the spring force gauge reading can be converted to the torque being applied to the load by the motor.
Let me know if this answers your question. Please note that this arrangement of a force-gauge-and-lever-arm plus an RPM gauge is known as a dynamometer and is the standard tool for measuring the horsepower being transferred by rotating shafts. You can find more examples of dynamometers on the web.