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Picking your perfect hydraulic motor won’t be that easy, but with the guidelines below you can start to select like a pro, says Jeff Klingberg.

We all wish selecting components for con-ag equipment was as easy as playing eenie-meenie-minie-mo. However, for each piece of compact equipment, application constraints like performance, efficiency, reliability, serviceability, productivity, control and cost have to be considered. This is especially true when designing hydraulic systems and selecting components such as hydraulic motors to operate the equipment’s drivetrain and attachments.

With two types of motors, three basic motor constructions, numerous variations on these designs and more than 20 hydraulic motor manufacturers and hundreds (if not thousands) of models, selecting the right motor for an application can lead to a confusing array of options and configurations.

Luckily, with some basic hydraulic motor knowledge, a buyer can certainly take much of the confusion out of the selection process.

First, motors are broken into two segments: high-speed, low-torque (HSLT) motors and low-speed, high-torque (LSHT) motors.

Generally, a LSHT motor’s speed can range between 0.1 and 1000rev/min depending on the motor and control system, whereas HSLT motors range between 1000 and 5000rev/min.

In this article, the focus will be on LSHT motors because these motors transmit a large amount of torque in a relatively small envelope - for example, a 5hp motor can be the size of a 350ml beer can.

Additionally, LSHT motors generate low noise levels and vibration, have a high-energy efficiency - in the 80 to 90% plus range - and operate smoothly at low speeds.

All of these attributes are ideal for con-ag and mobile equipment applications.

The combination of these LSHT motor characteristics gives it the ability to virtually eliminate the need for gearboxes in many applications.

Because of this, LSHT motors are great for con-ag equipment such as skid steers, mini excavators and their attachments.

LSHT motors generally come in three basic constructions: gear, piston or vane.

Gear motors are available in either external or internal gear configurations.

External motors will not be discussed as these motors are HSLT motors.

Internal gear motors - also known as gerotor, geroller or orbiting motors - have an inner (rotor) and outer (stator) gear.

The rotor has fewer teeth than the stator, giving it a large mechanical advantage.

Both gears move as the fluid passes through the motor, causing the rotor to orbit around the stator.

The drive coupling transmits the motion of the rotor to the output shaft.

Piston motors come in two basic designs: radial-piston and axial-piston.

These motors use reciprocating pistons to transmit the energy from the fluid to the drive trains or attachments.

Although there are numerous radial piston designs, the two most common are the telescoping piston and the multi-cam-lobe piston.

The telescoping piston design looks like the old airplane prop engines.

In this design, telescoping cylinders capture the pressurised fluid and push against an eccentric cam attached to the output shaft.

This design is not typically used in con-ag or mobile applications, except for large, heavy-duty applications like winches.

The multi-cam-lobe design is similar to an internal combustion engine whereby the pressurised fluid pushes against a series of pistons riding on a cam, which is attached to the output shaft.

A variation of this design is the rotating housing.

The cylinder block is stationary and the pistons ride on the cam in the housing causing it to rotate.

The rotating-housing-style motor is frequently used on wheel motors, forklifts etc.

As the name suggests, axial piston motors use reciprocating pistons axially aligned with the output shaft.

As the fluid enters the motor, the pistons push against an eccentric swashplate attached to the output shaft.

Most axial piston motors are HSLT; however, there are some manufacturers who make LSHT versions.

Generally, these are used on larger mobile and marine machinery, like large excavators and road planers.

However, at least one manufacturer makes an axial piston motor for use on skid steer and mini excavator drive trains among other applications.

This design includes an integrated gearbox.

Vane motors consist of a rotor that houses spring loaded vanes riding against a stationary cam ring.

As fluid enters the motor it is directed against the vanes, causing the rotor and output shaft to turn.

Just like axial piston motors, the majority of vane type motors are HSLT, however, there are some LSHT motors with maximum speeds up to 300rev/min.

Usually, these motors are used on drilling and/or auger applications.

So what about the performance, cost, efficiency, serviceability, controllability, reliability and popularity of all these motors?.

In general, all the motors have improved in all of these areas since the introduction of the LSHT motor in 1956.

As far as popular usage, many professionals in the industry are seeing the need for higher-pressure-capacity LSHT motors.

With that in mind, radial piston motors are being used in more drive applications in compact machines such as skid steers and mini excavators.

However, internal gear motors are popular as drive mechanisms on machines like scissor lifts, lawn mowers and a variety of other light- to medium-duty applications.

Gear motors are also being used more in the attachments for tool carriers like skid steers, and some gear motors are being used on skid steer and other con-ag equipment drivetrains.

And vane motors today are rarely used in mobile applications outside of augers and highspeed and low-speed drilling applications such as boring and mining equipment.

The average cost of hydraulic motors range from about $0.40 to more than $1.50 per lbf-ft of torque for a basic motor at the maximum torque level for a given motor size.

Options like brakes, speed sensors etc will increase these prices.

But be cautious about list prices - OEM pricing varies widely based on your usage and the application.

Traditionally, gear motors are the least expensive with piston motors having the highest initial cost.

However, their high initial cost may not truly reflect the expected overall cost over the equipment’s life.

Some of the piston motor manufacturers indicate their motors are competitive with gear motors in the 170 to 1000cm3 displacement range.

Reliability and serviceability go hand in hand.

Our research shows that the unofficial con-ag industry equipment minimum service life is 5kh.

All the motors are designed to achieve or exceed this life expectancy; however, operating conditions and duty cycle will significantly affect the life expectancy of the motors.

Following the manufacturer’s and industry’s recommended preventive maintenance guidelines will help to ensure this life expectancy is met.

Besides some throwaway gear-type motors, all the motor designs are capable of being repaired or rebuilt.

Some also offer the ability to add options, like brakes, speed sensors, change shafts, mountings and ports in the field.

Efficiencies are a function of flow (speed) and pressure.

Piston and vane motors with their balanced design - limits the effects of side loads on the motorare more efficient with piston motors having the highest efficiency.

Historically, gear motors have had the lowest mechanical and volumetric efficiencies.

This includes starting torque, which can be in the 50% range.

But, new gear motor designs are pushing efficiencies into the 90% range with no or minimal loss at startup.

Gear motors also lose mechanical and volumetric efficiency as they wear, however, piston and vane motors don’t, or minimally, lose efficiency because they are wear compensating.

Tortoise, hare or anywhere in between - all the motors are infinitely controllable through the use of closed circuit control systems.

It is essential that you follow the manufacturer’s specifications - especially at speed near zero.

At these speeds torque ripple and cogging can occur, this is particularly true for piston and gear motors.

Ultimately, whichever motor you select there needs to be a balance between cost, and your need for dependable, long life performance.

As Paul Klassy, Product Marketing Manager for Eaton Hydraulics puts it: ‘A motor has no value until the shaft turns and value increases as the shaft torque, speed, efficiency increases and noise decreases’.

There are a number of performance characteristics to consider when selecting a hydraulic motor.

First, the displacement is the volume of fluid required to turn the motor one revolution.

Common units of measurement are cubic inches or cubic centimetres per revolution.

Fixed displacement provides constant torque and speed is varied by controlling the amount of flow to the motor.

Dual Displacement - sometimes referred to as two-speed motors - provides multiple speeds with constant torque by controlling the motor’s internal valving.

Variable displacement provides variable torque and speed.

With the flow and pressure remaining constant, varying the displacement can vary the torque speed ratio.

The torque (T) is the rotational force generated by the motor at the output shaft as a function of system pressure and motor displacement, expressed in pound-inches (lbf-in), pound-feet (lbf-ft) or Newton-metres (Nm).

The breakaway torque is the torque required to get a stationary load moving.

The running torque can refer to the motor’s load or to the motor.

Load torque is the torque required to keep the load turning.

Motor torque is the actual torque a motor can generate to keep the load turning.

It considers the motor’s inefficiencies and is a percentage of its theoretical torque - approximately 90% of theoretical for gear, piston and vane motors.

Starting torque is the amount of torque a motor can generate to start a load turning, typically, expressed as a percentage of theoretical torque.

This ranges between 50 and 100% of theoretical depending on the motor design and manufacturer.

The mechanical efficiency is the actual torque divided by the theoretical torque.

Torque ripple is the variance between the minimum and maximum torque delivered at a given pressure during one revolution of the motor.

Speed is a function of motor displacement and the amount of fluid supplied to the motor.

The maximum motor speed is the speed at a specific pressure that the motor can sustain for a limited time without damage.

The minimum motor speed is the slowest, continuous, uninterrupted rotational shaft speed available from the motor.

Volumetric efficiency is the ratio of actual speed to theoretical speed with no leakage at a given input flow.

Although volumetric efficiency for LSHT motors can be 80 to 90% plus at peak performance, it can decrease to 50% at low speeds and vary with operating temperature and pressure.

Slippage refers to fluid passing through the motor without performing work.

Article Source:  http://www.engineeringtalk.com/news/flp/flp100.html