Hydrostatic pumps are positive displacement pumps while hydrodynamic pumps can be fixed displacement pumps, in which the displacement (flow through the pump per rotation of the pump) cannot be adjusted, or variable displacement pumps, which have a more complicated construction that allows the displacement to be adjusted.
Gear pumps (with external teeth) (fixed displacement) are simple and economical pumps. The swept volume or displacement of gear pumps for hydraulics will be between about 1 cm3 (0.001 litre) and 200 cm3 (0.2 litre). These pumps create pressure through the meshing of the gear teeth, which forces fluid around the gears to pressurize the outlet side. For lubrication, the gear pump uses a small amount of oil from the pressurized side of the gears, bleeds this through the (typically) hydrodynamic bearings, and vents the same oil either to the low pressure side of the gears, or through a dedicated drain port on the pump housing. Some gear pumps can be quite noisy, compared to other types, but modern gear pumps are highly reliable and much quieter than older models. This is in part due to designs incorporating split gears, helical gear teeth and higher precision/quality tooth profiles that mesh and unmesh more smoothly, reducing pressure ripple and related detrimental problems. Another positive attribute of the gear pump, is that catastrophic breakdown is a lot less common than in most other types of hydraulic pumps. This is because the gears gradually wear down the housing and/or main bushings, reducing the volumetric efficiency of the pump gradually until it is all but useless. This often happens long before wear causes the unit to seize or break down.
Total head and flow are the main criteria that are used to compare one pump with another or to select a centrifugal pump for an application. Total head is related to the discharge pressure of the pump. Why can't we just use discharge pressure? Pressure is a familiar concept, we are familiar with it in our daily lives. For example, fire extinguishers are pressurized at 60 psig (413 kPa), we put 35 psig (241 kPa) air pressure in our bicycle and car tires.For good reasons, pump manufacturers do not use discharge pressure as a criteria for pump selection. One of the reasons is that they do not know how you will use the pump. They do not know what flow rate you require and the flow rate of a centrifugal pump is not fixed. The discharge pressure depends on the pressure available on the suction side of the pump. If the source of water for the pump is below or above the pump suction, for the same flow rate you will get a different discharge pressure. Therefore to eliminate this problem, it is preferable to use the difference in pressure between the inlet and outlet of the pump.
The manufacturers have taken this a step further, the amount of pressure that a pump can produce will depend on the density of the fluid, for a salt water solution which is denser than pure water, the pressure will be higher for the same flow rate. Once again, the manufacturer doesn't know what type of fluid is in your system, so that a criteria that does not depend on density is very useful. There is such a criteria and it is called TOTAL HEAD, and it is defined as the difference in head between the inlet and outlet of the pump.
You can measure the discharge head by attaching a tube to the discharge side of the pump and measuring the height of the liquid in the tube with respect to the suction of the pump. The tube will have to be quite high for a typical domestic pump. If the discharge pressure is 40 psi the tube would have to be 92 feet high. This is not a practical method but it helps explain how head relates to total head and how head relates to pressure. You do the same to measure the suction head. The difference between the two is the total head of the pump.
Figure 25
The fluid in the measuring tube of the discharge or suction side of the pump will rise to the same height for all fluids regardless of the density. This is a rather astonishing statement, here's why. The pump doesn’t know anything about head, head is a concept we use to make our life easier. The pump produces pressure and the difference in pressure across the pump is the amount of pressure energy available to the system. If the fluid is dense, such as a salt solution for example, more pressure will be produced at the pump discharge than if the fluid were pure water. Compare two tanks with the same cylindrical shape, the same volume and liquid level, the tank with the denser fluid will have a higher pressure at the bottom. But the static head of the fluid surface with respect to the bottom is the same. Total head behaves the same way as static head, even if the fluid is denser the total head as compared to a less dense fluid such as pure water will be the same. This is a surprising fact.
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Variable flow Compensator hydraulic systems
This type of system is generally a higher flow pump and much more costly. This system generally requires an oil cooler and flow control valves in the valve stacks, due to higher flow rates. These type of systems are also generally lower pressure systems, typically 2000 PSI or less.Basic operation
These pumps have basically pistons attached to a plate. When there is a need for flow, the plate shifts increasing the stroke of the pistons and therefore it outputs more flow. This system always runs at full system pressure. When there is no oil flow there is no load because the plate with the pistons is vertical and there is no load. A relief valve is provided for safety but should never see any oil going over it in normal operation if set correctly. This type of system works well because it keeps constant pressure on the system and does not rely on P.O. checks holding pressure at the clamps. With this system you should still have P.O. checks incase the pump shuts off during the machining cycle.
Air over oil Pump systems
These systems are the cheapest of all systems. Depending on the brand and design you may have problems. They are very simple low volume/flow systems. They generally work well when using 2-4 valve stacks that are sequenced and do not require high volume of oil.Basic operation
Air is supplied to the system. Air will cycle an intensifier type cylinder and push oil from a reservoir into the hydraulic system. Pressure is adjusted by adjusting the incoming air flow.
High Pressure coolant systems buying considerations, efficiency, and coolant system applications
High pressure coolant can be very beneficial for almost any job, some much more then others. The benefits of the high pressure coolant usually out weigh the initial cost when it comes to tooling savings, cycle time and finish. Tooling will last longer, speeds will be increased especially when drilling or boring. High pressure coolant will keep the part from heating up by transferring the heat into the coolant, away from the tool and part. Therefore, holding tighter tolerances, keeping your chips consistent and heat free. Learn to turn, mill, and drill faster with high pressure coolant.Benefits of high pressure coolant
- Often drastic cycle time reduction (20-70%)
- Increased speeds in hole production
- Eliminate heat related failure of insert
- Reduced chip welding and "built-up edge" in aluminum machining
- Better chip control in Low Carbon Steels
- Improved quality and speed of Threaded Holes
- Reduce or eliminate work hardening from peck drilling
- Improved tapping in tough materials
- Increased life of expensive Custom Tooling
- Improved Auto-loading by eliminating chip problems
- Increased tool life in Abrasive Materials
The basic hydraulic pump system
The basic unit will contain a .5-1.0 GPM pump which will be sufficient for the majority of fixtures unless you are using larger volume of clamps or multiple operations at once. The size of the motor depends on the pressure needed - 1,3,5(HP) horse power. The pump reservoir size is usually a 1, 3 or 5 gallon. The pump system should also contain an oil low level and HI-temperature switch for protection. Pump specs for volume can be calculated by figuring out the total volume of the clamps and figuring the speed at which the chamber fills up assuming .5 gal per minute.
The hydraulic valving for a double acting fixture (can also be used for single acting)
- Directional control valves- Poppet (Least expensive,preferred for low flow small systems) or spool type (can handle more contaminants in oil)
- P.O. check valves on A+B
- Tapping Block for pressure switches on A and B
- Pressure switches on A+B (Sometimes only A port I'll explain later)
- Dump valve
- Pressure reducing valve (not usually needed)
- Flow controls (not usually needed- used usually with small volume work supports)
Hydraulic pressure switch considerations
Hydraulic pressure switches may not be necessary on B port if it is a single actuating fixture. If it is a single actuating fixture with a robot load, I would recommend them to confirm that there is no residual pressure left on the clamp side. Then causing the robot to collide with the clamps.Also, if it is a manual operation only the PLC may run the unclamp solenoid for a specified amount of time instead of waiting for the unclamp pressure switch to be made. However; if there is any safety concerns during the unclamp cycle where the operator can be injured, this may not be a good idea.
