There are two basic measurements with hydraulic power. The pressure of the fluid, commonly expressed in units of pounds per square inch pressure, represents the force of the system. The second measurement is flow rate, normally expressed in units of gallons per minute, and this measures the amount of hydraulic fluid being used per unit of time. The horsepower produced is a function of the pressure times the flow rate. The same horsepower can be developed by a low pressure and large flow rate or a high pressure with a low flow rate.
In a complete hydraulic drive system, the basic source of power must still be an electric motor or an internal combustion engine. This basic power source operates a hydraulic pump which draws hydraulic fluid from a reservoir and places the fluid under pressure. The pressurized fluid is then transmitted by pipes or hoses to a hydraulic motor, which transfers the hydraulic pressure back to mechanical work, driving a gear reducer, a pinion to a ring gear, or the bull-wheel itself. The hydraulic fluid, freed of the pressure, flows out of the hydraulic motor back to the reservoir.
The question may well be asked—Why hydraulic, if all the system does is change mechanical energy into hydraulic energy at one end and then change it back to mechanical energy at the other end? There are two basic features of the hydraulic system which make it attractive for lift usage. First, the transfer of the power in the hydraulic system between pump and hydraulic motor can be accomplished by simply running hydraulic lines (piping or flexible hose). The need for mechanical couplings, right angle gears, line shafting, etc. is eliminated in this portion of the drive. Secondly, the speed of the system is a direct function of the flow rate of the hydraulic fluid. Speed control is but a question of how much hydraulic fluid flows through the hydraulic motor.
There are two additional features of a hydraulic system often used in lift applications. One advantage of the hydraulic system that is sometimes used is the ability to use a high speed pump to operate a low speed hydraulic motor. In this manner, one or two gear reductions may be eliminated. A pump may operate at 1750 r.p.m., but a hydraulic motor may be chosen that has an output of less than 100 r.p.m., so that the system is effectively a speed reducer. The second feature sometimes used in chair lift applications is the “overspeed braking” that can be incorporated in the hydraulic system. When the lift tends to over-run, the system can “reverse itself” so that the hydraulic motor becomes a pump and the rate of hydraulic flow through this unit is controlled by a “flow-control” valve.
Unfortunately, a hydraulic drive system is not as simple as a pump, hydraulic motor, hydraulic lines, and a reservoir. Neither the pump nor the hydraulic motor are 100 per cent efficient, and pressure losses do occur in the hydraulic lines. All of these losses in energy are reflected in heat—so that usually a heat exchanger (oil cooler) is required to be added to the system. The two common types of heat exchangers are the air-cooled exchanger and the water-cooled exchanger. For cold weather and exterior applications the air-cooled exchanger is normally used.
The hydraulic fluid must be kept within certain ranges of viscosity to function correctly with the other hydraulic equipment. The heat exchanger mentioned above controls the upper limits of hydraulic fluid temperature, but frequently some form of heater must be provided to maintain an acceptable minimum temperature for cold morning start-up. Immersion heaters in the oil reservoirs are frequently used to maintain this minimum temperature. The problem of maintaining a minimum temperature in the hydraulic lines and at the hydraulic motor is more complicated, and each individual system requires specific attention.
For the hydraulic fluid to function properly as it flows through the pump, hydraulic motor and valves, it must be free from dirt or other foreign matter. Tolerances in the hydraulic equipment are close, seals are incorporated in most components, and to maintain efficiency and keep leakage to a minimum, these components cannot be allowed to be exposed to foreign material in the “wet” parts.
To protect, control and “read” the system, a variety of gauges, valves and other accessories are usually incorporated. Check valves are usually found in the system, normally to prevent the reverse flow of the fluid under periods of idleness or abnormal conditions. These permit the flow of the fluid in one direction, but close to prevent flow in the reverse direction. Valves may be installed to limit or regulate the maximum pressure in any part of the system. Valves can be installed which will regulate or limit the flow rate of the hydraulic fluid. Pressure gauges will show the “force” in the system. Temperature gauges will indicate the temperature of the hydraulic fluid, and thermostats can be installed to maintain the necessary amount of heating or cooling that the fluid may require.
| Horsepower | Flow Rate, Gallons per minute — 500 p.s.i. | Flow Rate, Gallons per minute — 1500 p.s.i. | Flow Rate, Gallons per minute — 2500 p.s.i. | Hydraulic Line Size, Inside Diameter—Inches — 500 p.s.i. | Hydraulic Line Size, Inside Diameter—Inches — 1500 p.s.i. | Hydraulic Line Size, Inside Diameter—Inches — 2500 p.s.i. |
|---|---|---|---|---|---|---|
| 10 | 34 | 11 | 7 | 1-3/16 | 11/16 | 9/16 |
| 25 | 86 | 29 | 17 | 1-7/8 | 1-1/8 | 7/8 |
| 50 | 171 | 57 | 34 | 2-11/16 | 1-9/16 | 1-3/16 |
| 75 | 257 | 85 | 51 | 3-1/4 | 1-7/8 | 1-1/2 |
| 100 | 343 | 114 | 69 | 3-3/4 | 2-3/16 | 1-11/16 |
| 150 | 514 | 171 | 103 | 4-5/8 | 2-11/16 | 2-1/16 |
| 200 | 686 | 228 | 137 | 5-5/16 | 3-1/16 | 2-3/8 |
To carry the hydraulic fluid from reservoir to the hydraulic components and back to the reservoir, hydraulic lines and fittings are required. The specific design of these lines depends upon the operating pressure of the system and the flow rate of the hydraulic fluid. The speed of travel of the fluid through the lines must be kept low to minimize losses in pressure in the lines. High factors of safety in these lines are necessary to provide protection against a line failure which would result in loss of hydraulic fluid. Since mechanical vibrations and hydraulic shock loads may be present, provisions to prevent the hydraulic lines from becoming subject to these “outside” loads must be provided. Flexible joints or flexible lines are usually employed. Connections must be structurally sound as well as being leak-proof. The choice between operating a system at a relative low pressure with high volumes of hydraulic fluid flow as opposed to using high pressures with smaller flows is a complex problem, both as to choice of equipment and sizing of hydraulic lines. Table 1 shows the flow rates necessary to develop certain horsepowers and the recommended inside diameter of a hydraulic line required to transport this fluid at a velocity of ten feet per second.
Certain lift drive systems have incorporated single hydraulic (or fluid) devices and should not be confused with a complete hydraulic drive. The most simple of these units is the fluid coupling, which is a self-contained unit which permits a slip between the drive motor or engine and the drive output at low speeds, but at rated speed reduces this slip to nearly zero. The primary purpose of such a coupling is to provide for cushioned starts. Hydraulic speed-change units are also used where the entire mechanical and hydraulic configuration is housed in a single unit (excepting an auxiliary pump and heat exchanger). Such a unit provides for a “cushioned” start and also speed control.
No attempt is made here to discuss the various types of hydraulic equipment or fluids. Hydraulics permit a wide selection of arrangements. The most important consideration of such a system is that it must function as a unit; each part and component must be selected to be compatible with every other part or component. The overall system must have the extreme dependability that is essential to ski lift operation.
