A hydraulic system can be used for holding and positioning the parts to be welded during a welding operation. It is a typical example of how fluid power can be used in manufacturing and production operations, to reduce the overall costs and to increase production.
This application requires a sequencing system for fast and positive holding. This is accomplished by placing a restrictor (sequence valve) in the line leading to the second cylinder, as shown in Figure 9.5. The first cylinder extends to the end of its stroke.
Oil pressure then builds up, overcoming the restrictor setting and the second cylinder extends to complete the ‘hold’ cycle. This unique welding application of hydraulics was initiated to increase productivity.
Most overhead trams require haulage or tow cable to travel up and down steep inclines. A 22-passenger, 12 000 pound (around 5000 kg) hydraulically powered and controlled tram is shown in Figure 9.2.
It is self-propelled and travels on a stationary cable. Since the tram moves instead of the cables, the operator can easily start, stop and reverse a particular car completely independent of any other car in the tram system.
Integral to the design of the sky tram drive is a pump (driven by a standard 8 cylinder gasoline engine) which supplies pressurized fluid to four hydraulic motors. Each of the four motors drives two friction drive wheels. Eight drive wheels on top of the cables support and propel the tramcar. On steep inclines, while a higher driving torque is required for ascending, a higher braking torque is required during descent. Dual compensation of the four hydraulic motors provides efficient proportioning of the available horsepower to meet the variable torque demands.
Figure 7.25 shows an air-cooled heat exchanger.
The hydraulic fluid to be cooled is pumped through the tubes that are finned. As the fluid flows through the tubes, air is blown over them. This takes away the heat from the tubes. A fan driven by an electric motor is incorporated in the heat exchanger to provide air for cooling. The heat exchanger shown above, uses tubes which contain special devices called turbulators whose function is to mix the warmer and cooler oils for better heat transfer.
Advantages associated with air-cooled heat exchangers are:
1. Substantial cost reduction because of the use of air for cooling purposes, as compared with water
2. Lower installed costs
3. Possibility of the dissipated heat being reclaimed.
Disadvantages of air-cooled heat exchangers are:
1. Relatively larger in size
2. High noise levels
3. Higher installation costs.
One of the most important industrial applications of accumulators is in the elimination or reduction of high-pressure pulsations or hydraulic shocks.
Hydraulic shock (or water hammer, as it is frequently called) is caused by the sudden stoppage or deceleration of a hydraulic fluid flowing at a relatively higher velocity in the pipelines. This hydraulic shock creates a compression wave at the location of the rapidly closing valve. This wave travels along the length of the entire pipe, until its energy is fully dissipated by friction. The resulting high-pressure pulsations or high-pressure surges may end up damaging the hydraulic components.
An accumulator installed near the rapidly closing valve as shown in Figure 7.24 can act as a surge suppressor to reduce these high-pressure pulsations or surges.
In this application (Figure 7.23), the accumulator acts as a compensator, by compensating for losses due to internal or external leakage that might occur during an extended period of time, when the system is pressurized, but not in operation.
The pump charges the accumulator and the system, until the maximum pressure setting on the pressure switch is obtained. When the system is not operating, it is required to maintain the required pressure setting, to accomplish which the accumulator supplies leakage oil to the system during a lengthy period of time. Finally when the system pressure falls below the minimum required pressure setting, the pump starts to automatically recharge the system. This saves electrical power and reduces heat in the system.
This is one of the most common applications of an accumulator. In this application, the purpose of the accumulator is to store the oil delivered by the pump during the work cycle. The accumulator then releases the stored oil on demand, to complete the cycle, thereby serving as a secondary power source to assist the pump. In such a system where intermittent operations are performed, the use of an accumulator results in reduced pump capacity.
Figure 7.22 outlines this application with the help of symbols.
In this application, a four-way valve is used in conjunction with an accumulator. When the four-way valve is manually actuated, oil flows from the accumulator to the blank end of the cylinder. This extends the piston until the end of the stroke. When the cylinder is in a fully extended position, the pump charges the accumulator. The four-way valve is then de-activated to retract the cylinder. Oil flows from the pump and the accumulator to retract the cylinder rapidly. This is how an accumulator can be used as an auxiliary power source.
The bladder-type accumulator contains an elastic barrier between the oil and gas as shown in the cross-sectional view in (Figure 7.20).
The bladder is fitted to the accumulator by means of a vulcanized gas-valve element that can be installed or removed through the shell opening at the poppet valve. The poppet valve closes the inlet when the bladder is fully expanded. This prevents the bladder from being pressed into the opening. A shock-absorbing device, protects the valve against accidental shocks, during a quick opening.
The greatest advantage with these accumulators is the positive sealing between the gas and oil chambers. The Ughtweight bladder provides a quick pressure response for pressure regulation as well as applications involving pump pulsations and shock dampening.
Figure 7.21 illustrates the functioning of a bladder-type accumulator.
The hydrauhc pump delivers oil to the accumulator and deforms the bladder. As the pressure increases, the volume of gas decreases. This results in the storing of hydraulic energy. Whenever additional oil is required by the system, it is supplied by the accumulator even as the pressure in the system drops by a corresponding amount.
This accumulator consists of a cylinder containing a freely floating piston with proper seals, as illustrated in Figure 7.17.
The piston serves as a barrier between the gas and oil. A threaded lock ring provides a safety feature that prevents the operator from disassembling the unit while it is precharged.
The main disadvantage of piston-type accumulators is that they are very expensive and have size limitations. In low-pressure systems, the piston and seal friction also poses problems. Piston accumulators should not be used as pressure pulsation dampeners or shock absorbers because of the inertia of the piston and the friction in the seals.
The principle advantage of the piston-type accumulator lies in its ability to handle very high- or low-temperature system fluids, through the utilization of compatible O-ring seals.
The non-separator type consists of a fully enclosed shell containing an oil port at the bottom and the gas-charging valve at the top. The valve is confined to the top and the oil to the bottom of the shell. There is no physical separator between the gas and oil, and thus the gas pushes directly on the oil.
The main advantage of this type of accumulator is its ability to handle a large volume of oil. However, its disadvantage lies in the fact that the oil tends to absorb gas due to the lack of a separator. A cross-section of a non-separator type accumulator has been illustrated in Figure 7.16.
A gas-loaded accumulator must be installed vertically to keep the gas confined to the top of the accumulator. It is not recommended for use with high-speed pumps as the entrapped gas in the oil may cause cavitation and damage the pump. The absorption of gas in the oil also makes the oil compressible, resulting in spongy operation of the actuators.
A strainer is a device made of wire mesh screens, which seek to remove large solid particles from a fluid. As part of standard engineering practice, strainers are installed on pipelines ahead of valves, pumps and regulators, in order to protect them from the damaging effects of fluid and other system contaminants.
A common strainer design uses two screens, cylindrical in shape. One cylinder is inside the other and the two are separated by a small space. The outer cylinder is a coarse mesh screen and the inner one is a fine mesh screen. The fluid first passes through the coarse mesh screen and filters the larger particles. It then passes through the fine mesh screen, which blocks the smaller particles. Figure 7.11 shows the cross-sectional view of a typical strainer.
The bottom of the strainer serves as the sump (or pot) for the solids to collect. The strainer can be cleaned out easily at intervals, by two different procedures:
1. The cleanout plug can be removed and the pressure in the line can be used to blow the fixture clean.
2. The large retaining nut at the bottom is to be removed for pulling the mesh out of the strainer in order to clean it and putting it back in line.