Learning Objectives
By the end of this chapter, students should be able to:
– Differentiate between a hydraulic motor and a hydraulic pump.
– Identify various applications of hydraulic motors in fluid power.
– Discuss different classifications of hydraulic motors.
– Explain the construction and operation of gear, vane, and piston motors.
– Describe the various types of limited-rotation motors.
– Explain different efficiency terms related to hydraulic motors.
– Assess the performance parameters of systems utilizing motors.
1.1 Introduction
Automotive Lift Repair Orlando Hydraulic motors are classified as rotary actuators, but the term “rotary actuator” specifically refers to devices that have restricted rotation of less than 360 degrees. A hydraulic motor converts fluid power into rotary power, essentially transforming fluid pressure into torque. The torque generated is determined by the input pressure level and the resisting torque at the output shaft. Conversely, a hydraulic pump transforms mechanical force and motion into fluid power. It’s important to note that a hydraulic motor does not function as a hydraulic pump when operated in reverse; a motor designed for hydraulic operation may perform poorly as a pump under certain conditions.
1.2 Applications
Automotive Lift Repair Orlando Hydraulic motors are widely utilized in various industries due to their ability to be directly applied to work. Additionally, they offer design flexibility while reducing the bulk and weight associated with mechanical and electrical power transmission. When combined with pumping units, these applications form what is known as hydrostatic transmission.
A hydrostatic transmission converts mechanical power into fluid power and subsequently reconverts it into shaft power. Its benefits include the ability to transmit power to remote locations, infinitely variable speed control, self-overload protection, reverse rotation capabilities, dynamic braking, and a high power-to-weight ratio. Common applications include material-handling equipment, farm tractors, railway locomotives, buses, lawn mowers, and machine tools.
New applications for Automotive Lift Repair Orlando hydrostatic transmissions continue to emerge, encompassing farm implements, road machinery, material-handling equipment, numerical control machines, high-performance aircraft, military uses, and specialized machinery. Many vehicles, including automobiles, railway locomotives, and buses, utilize hydrostatic transmissions.
1.3 Comparison Between a Hydraulic Motor and an Electric Motor
Automotive Lift Repair Orlando Hydraulic motors and electric motors have distinct differences. Electric motors cannot be stopped instantly, nor can their direction of rotation be reversed immediately due to the air gap between the rotor and stator and the weak magnetic field. In contrast, hydraulic motors can be stalled for any duration, and their direction of rotation can be quickly reversed. They can also have infinitely variable rotational speeds without impacting torque and can be braked instantly, exhibiting high torque capacities.
Electric motors tend to be heavy and bulky, while hydraulic motors are more compact, occupying approximately 25% of the space and weighing around 10% of equivalent electric motors. The moment of inertia-to-torque ratio for electric motors is nearly 100, whereas for hydraulic motors, it is approximately 1.
1.4 Classification of Hydraulic Motors
Hydraulic motors can be categorized into two types: high-speed low-torque motors and low-speed high-torque motors. In contrast, low-speed high-torque motors utilize a differential gear arrangement to reduce speed and increase torque. Based on the mechanism used for shaft rotation, hydraulic motors are classified into:
1. Gear motors
2. Vane motors
3. Piston motors, which include:
– Axial piston-type motors
– Radial piston-type motors
Automotive Lift Repair Orlando Gear motors are the least efficient and most tolerant to dirt, with the lowest pressure ratings. Piston motors are the most efficient, least tolerant to dirt, and possess high pressure ratings. Vane and piston motors can be either fixed or variable displacement, while gear motors are available only in fixed displacement.
1.5 Gear Motors
Gear motors generate torque through hydraulic pressure acting on the surface area of the gear teeth. In a typical design, one gear is connected to an output shaft, while the other serves as an idler gear. Pressurized oil enters the motor’s inlet port, applying pressure to the gear teeth, which causes both the gears and output shaft to rotate. The pressure continues to build until sufficient torque is achieved to turn the output shaft against the load. However, this results in significant side load on the motor bearings due to the concentrated hydraulic pressure, which can limit bearing life.
Most gear motors are bidirectional; reversing the flow direction changes the rotation direction. Like gear pumps, gear motors have fixed volumetric displacement. Due to the pressure differential between the inlet and outlet, considerable side loads develop on the shaft and bearings. Automotive Lift Repair Orlando Gear motors typically operate at a maximum of 150 bar and 2500 RPM, with flow capacities up to 600 LPM. Although simple in design and robust against dirt, gear motors are less efficient than vane or piston motors and have higher leakage rates. They are generally unsuitable as servo motors, although internal gear designs can operate at higher pressures, speeds, and displacements than external gear motors.
1.6 Vane Motors
Vane motors consist of a circular chamber housing an eccentric rotor with multiple spring or pressure-loaded vanes. Fluid entering the inlet port exerts greater force on the upper vanes, causing the rotor to rotate counterclockwise. Close tolerances between the vanes and the chamber ensure high efficiency.
The displacement of a vane motor depends on its eccentricity. Unbalanced vane motors experience significant radial loads on the shaft bearing because inlet pressure is concentrated on one side of the rotor. In contrast, balanced vane motors eliminate this issue by using a double-lobed ring with ports located diametrically opposite each other. This design cancels out side forces, with one inlet and one outlet port being external. While balanced vane motors are reliable for open-loop control, they tend to have more internal leakage than piston-type motors, making them less suitable for servo applications.
1.7 Piston Motors
Automotive Lift Repair Orlando Piston motors can be classified based on the arrangement of the piston within the cylinder block and the drive shaft as follows:
1. Axial piston motors
2. Radial piston motors
They can also be categorized according to displacement into:
1. Fixed-displacement piston motors
2. Variable-displacement piston motors