Tractor Design & Testing 3(2+1)

Lesson 17. Hydraulic system design considerations

1. Hydraulic Pumps:

The pump is a mechanical device that converts energy from the prime mover (electric motor or engine) to fluid power energy. Pumps are rated by pressure and flow. Pressure is typically measured in psi or bar. Flow is measured in liters per minute (l/m) and is a result of volumetric displacement x rpm. Currently there are three types of hydraulic pumps used; gear pumps, axial piston pumps and vane pumps. All are positive displacement pumps and each has its own limitations and advantages.

(i) Gear Pumps

When new; most gear pumps have a practical efficiency of 85% to 90% and will decrease over time. New piston and vane pumps’ efficiencies are somewhere around 90 to 95% and too will lose efficiency as the pump wears. Industrial standards for pump lifetimes are between 8000 to 10000 hours of operation. All pumps work by mechanically squeezing the fluid between mating surfaces. In gear pumps it is the meshing of the gears pushing against the pump housing wall, in piston pumps it is the piston pushing in the piston silo against a valve plate, and in vane pumps it is the vanes pushing against the pump housing wall. In all cases the pumps are dependent on maintaining lubricity between mating surfaces to prevent material on material wear. All three types of pumps are sensitive to fluid contamination, some more than others. Gear pumps are the least sensitive while piston and vane pumps are the most sensitive. All pumps are also sensitive to fluid cavitations or air bubbles that form in the fluid. These bubbles when compressed will explode creating pitting and pockets in the pump material.

Pumps are designed to have a controlled rate of internal leakage to provide lubrication to the rotating parts and to provide pump cooling. As pumps wear over their lifetimes, the ability to produce pressure and flow diminishes along with ever increasing internal leakage. From a practical standpoint, when pump efficiencies start dropping below 80 to 85%, depending on the type, it is time to replace the pump or rebuild it. Petroleum based mineral oils are mostly used as hydraulic oil. Hence, hydraulic pumps have been designed around using mineral oil based fluids.

Gear pumps are the most commonly used hydraulic pumps. They are less mechanically complicated and are cheaper to manufacture. Gear pump housings come in a variety of materials from cast aluminum to 316 L stainless steel. The manufacturer decides on the best combination of materials for the target market. Even though gear pumps are less efficient than piston or vane pumps, the advantage of low cost and contamination robustness make them a good choice for the majority of applications, from mobile to portable to fixed sites. Gear pumps are used in applications that are ≤3000 psi (210 bar). Some gear pumps achieve pressures in excess of 3000 psi but this is the exception rather than the rule.

(ii) Vane pumps:

One great advantage of a vane pump provides is that it runs quieter than either gear pumps or piston pumps. Also, vane pumps exceed gear pumps in pressures up to ≤ 4000 psi. Of the three pump types vane pumps tend to be the most dirt sensitive. For some mobile applications where noise is an application consideration vane pumps are making a resurgence in use.

(iii) Axial piston:

These pumps are capable of routinely achieving continuous running pressures > 5000 psi (345 bar). Some units are now approaching 7500 psi (520 bar). A good share of piston pumps are used as hydrostatic units where the system return flow is fed back into the inlet side of the pump without the use of large hydraulic reservoirs. The higher pressures, speeds and absence of large reservoirs are a weight saving strategy. These systems run at slower speeds and pressures from 1500 to 2000 psi. Piston pumps come in two basic types; fixed displacement and variable displacement. A fixed displacement piston pump is just that. Output flow can only be varied by pump speed whereas in a variable displacement piston pump, the pump volume can be adjusted; thus varying the flow without varying the speed. Variable displacement pumps have a greater number of moving parts making lubrication critical. The ability to adjust pump volume is clearly an energy saving feature and positively affects the pump lifetime. This particular pump is to be used in open system applications.

2. Motors:

Motors are essentially pumps working in reverse. They convert the fluid energy back into mechanical energy. In the case of rotary actuators (motors) the energy is converted to rotational force (torque). Force is a function of pressure and speed is a function of flow. High pressure high torque, high flow high speed. Motors by and large are fixed displacement. Motors are classified into one of four types; Low Speed High Torque (LSHT), High Speed Low Torque (HSLT), Low Speed Low Torque (LSLT), and High Speed High Torque (HSHT). The two most common types are LSHT and HSLT. Just as in pumps there are four common materials; aluminum, cast iron, cast steel and stainless steel. Although pumps are designed as gear, vane, and axial piston; motors also have another type of design, orbital. The orbital motors are exclusively LSHT and are the most popular design of hydraulic motor in use today. Unlike pumps which can be, quite often, placed in more controlled environments, such as an equipment room, motors can and are subjected to whatever the environment offers. The robustness of the motor is critical to resisting the expected environmental conditions. Other than hoses and fittings, rotary and linear actuators are the chief sources of fluid leakage. Thus, not only must motors keep the prevailing environment out, but must keep the hydraulic fluid in. OEM equipment designers carefully select the type of motors to be used not only on the work to be done, but also on the expected operating environmental conditions.

3. Cylinders:

Cylinders are actuators that convert fluid power force into linear mechanical motion. The motion may be pushing, pulling, lifting, or lowering. Although the construction of a cylinder is fairly simple, having only one moving part, other considerations are critical to application suitability. Cylinders are made of a number of materials; plastics and composites, steel, aluminum and stainless steel. Cylinders are either double acting or single acting. Double acting means that fluid can be applied to either side of piston, rod end or tail end. Single acting cylinders are capable of applying fluid force to only one end, usually the tail end, and retract by means of some other force such as gravity or springs. To contain and direct the force sealing is critical at the piston and rod. Cylinders are applied based on force and speed of actuation. Cylinders are sensitive to force media viscosities. Where extremely high speed is required with low to medium force; pneumatic cylinders are usually applied. Since air or gases have low viscosities but high compressibility or low bulk modulus; they are limited to the high speed low force applications. High force and low to medium speeds is where hydraulic cylinders are applied. A basic rule is that the higher the speed the lower the fluid viscosity and/or the greater the flow.
 
Just as with motors, cylinders are exposed to a variety of environmental conditions. Cylinders are too challenged with maintaining both internal and external integrity. As a component, cylinders represent the single greatest threat to leakage. Systems with a large quantity of cylinders are a good application for fluids that are eco-friendly.

4. Reservoirs:

The reservoir can be constructed of a variety of materials including aluminum, plastics and composites, mild steel, and stainless steel. As a fluid storage tank it must assure that the fluid is protected from the environmental elements, maintain the fluid integrity and provide a ready supply to the pump. Other functions of the reservoir include providing fluid cooling through the process of thermal radiation, maintain positive pump head pressure, and suppression of fluid turbulence. The material that a reservoir is constructed from affects other system operational parameters. Reservoirs constructed of aluminum, mild steel, and stainless steel provide the benefit of fluid cooling through the process of thermal radiation to the external atmosphere. In open loop hydraulic systems and to assure that the reservoir is able to supply fluid to the pump and provide adequate cooling, it is a rule of thumb to size it a minimum of 3 times the system flow. For example: systems with flow requirements of 40 lpm would have a reservoir of 120 litres minimum. In theory the fluid would have 40 seconds dwell time in the reservoir for every 20 seconds under load. The 40 seconds will provide sufficient dwell time to cool the fluid and suppress the turbulence under normal conditions. However, when reservoirs are constructed of plastic or composites and a heat exchanger is not used, reservoir sizing may be 4 to 5 times system capacity. This allows for longer dwell times. It is almost imperative that with plastic or composite constructed reservoirs that an external heat exchanger should be employed. As far as construction materials, aluminum tanks should be closely investigated.

Tank turbulence from return hydraulic fluid must be controlled. Uncontrolled tank fluid turbulence creates conditions that can result in pump cavitation when the pump is installed in the reservoir. Reservoir breathers should be of a filter breather type. A filter breather not only helps slow water evaporation but also controls the ingression of air born contaminants. In a non-pressurized tank the breather performs an important function by allowing for the equalization of atmospheric pressures between the inside of the tank and the outside ambient pressure.