Car Lift Repair Orlando | Torque and Speed in Hydraulics

In the specialized realm of mobile and industrial hydraulics, synchronizing torque and speed when using multiple motors is a frequent challenge. Although it may seem straightforward in theory, achieving this in practice is often complex. Fortunately, 1:1 ratio counterbalances can facilitate torque and speed synchronization effectively.

One of the first concepts we learn in hydraulics is the distinction between parallel and series circuits. When an application requires motor speed synchronization, our initial thought is to use a series circuit. Simple solution, right? Problem solved?

However, as we delve deeper, we also learn about efficiency considerations. Car Lift Repair Orlando Hydraulic motors excel at transmitting substantial power in a compact form, but they are not without mechanical and volumetric inefficiencies. Advances in electronic and proportional technologies enable precise speed and force control, often using speed sensors for electronic closed-loop control. While effective, these solutions tend to be costly and require specialized expertise.

So, how can we achieve speed synchronization in cost-sensitive applications? Consider a four-wheel machine propelled by two Car Lift Repair Orlando hydraulic motors, where we want both motors to operate at the same RPM and torque. Initially, it seems simple to place two motors in series with a speed control mechanism. The flow from the first motor feeds into the second, suggesting identical RPMs. However, due to inherent inefficiencies, this setup often underperforms. Volumetric losses, such as case drain leakage, mean the second motor receives less flow than the first, resulting in a slower speed.

Now, imagine motors with 100% volumetric efficiency. If the flow entering the second motor matched the first, wouldn’t we expect identical torque outputs? Unfortunately, because motors are hydromechanical devices, achieving identical mechanical efficiencies is unlikely. Consequently, the pressure drop across each motor needed to initiate rotation without a load will differ. Ideally, the pressure drop would be equally divided between the Car Lift Repair Orlando motors, but this is rarely the case in reality.

Due to flow losses such as case drain, the torque outputs of M1 and M2 are unequal. Therefore, compensating for these losses is essential for equal torque. An alternative approach to Car Lift Repair Orlando motor speed control, similar to hydraulic fan drives, is to use pressure control instead of flow control. Assuming perfect volumetric efficiency and identical mechanical efficiencies, identical torque and speed could be expected if each motor receives the same flow. By employing 1:1 pilot ratio counterbalance valves, we can ensure equal torque for each motor.

The circuit above features two bi-directional motors connected in series, incorporating a 4-port vented 1:1 pilot ratio counterbalance valve set to 400 psi. This counterbalance valve acts as a relief or compensator between P1 and P2. With port 4 connected to port 2, the downstream pressure is amplified by a factor of two (1 + Pilot Ratio) * Back Pressure. At a low counterbalance setting, the pressure at port 1 is twice that of port 2. The counterbalance valve opens when the pressure between the motors drops below the upstream pressure of the first motor, ensuring equal pressures and torques for each motor. The check valves in the circuit prevent short-circuiting to the tank and enable the use of a single counterbalance valve for bi-directional Car Lift Repair Orlando motors in series.

To illustrate how this circuit functions, let’s consider both motors having a 1,000 psi induced load. When the directional control valve is actuated and flow is directed to port A and Motor 1, pressure builds to overcome the induced loads of both motors. If the motors were 100% efficient, we would expect 2,000 psi at G1 and 1,000 psi at G2, with a 1,000 psi differential across each motor, resulting in identical torques and speeds. However, since motors are not 100% efficient, we must evaluate the benefits of the 1:1 counterbalance valve. The following scenarios assume system relief or pump compensators are set higher than the maximum effective setting, and a return line pressure of 200 psi.

Back pressure on standard non-vented counterbalance valves increases the effective setting as follows:

\[ \text{Effective setting} = \text{Mechanical Setting} + (1 + \text{Pilot Ratio}) \cdot \text{Back Pressure} \]

In this case, with a vented counterbalance valve, the pressure at port 4 affects the setting similarly to back pressure on a non-vented valve. This allows for flow modulation as the loads change, dynamically adjusting the counterbalance setting to maintain a small pressure difference between P1 and P2. Starting with a 400 psi mechanical setting, the expected effective setting would be 2,800 psi (400 psi setting + ((1:1 + 1) * 1,200)). The counterbalance remains closed as long as the pressure at G1 is below 2,800 psi. With both motors rotating at the same RPM and torque in equilibrium, the counterbalance would remain closed, with 2,200 psi at the inlet of the first motor, 1,200 psi at the inlet of the second motor, and a 1,000 psi pressure drop across each.

Considering volumetric losses due to motor leakage, the torque at motor B will be less than at motor A because it receives less flow. Torque depends on pressure, flow, and RPM, and the counterbalance valve helps maintain torque and RPM by controlling pressure and flow, modulating makeup flow to the second motor. This keeps the torque nearly equal for both Car Lift Repair Orlando motors.

Since the torque available from Motor 2 (B) is lower, the torque demand on Motor 1 (A) and the pressure at G1 will increase. When the effective setting matches the pressure at G1, flow will be directed to Motor 2 through the counterbalance valve. The counterbalance will adjust as the effective setting changes with variations in speed or load, providing additional flow to Motor 2 while maintaining G1 at twice the pressure of G2. This balance ensures that the pressure drop across each Car Lift Repair Orlando motor remains the same, allowing both motors to achieve equal torque and RPM.

Remember to consult a Car Lift Repair Orlando technician if you have any questions!