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The typical fixture hydraulic systems and operation

The hydraulic system used will obviously depend on many variables, but here is what I have found to be the most likely and useful configuration.

Fixed displacement pump hydraulic system with dump valve

Basic hydraulic system operation

Hydraulic pump sits at idle dumping over a dump valve to unload the pump. Clamp command is given and the dump valve is energized to prevent oil flowing directly to the tank. Once the clamps have been completed and pressure is at the maximum, the pressure switch actuates and shuts off the clamp solenoid and dump valve. Therefore, unloading the oil from the pump to tank. The clamps maintain pressure because of a P.O. check installed in the valve stack. If the P.O. check leaks or there is excessive leakage the hydraulic pump can be set up to perform a recharge though this is often not required. If a recharge is required the PLC will control the logic and dual setting pressure switches are needed for safety check. The first pressure switch set point will control the recharge point and the second will be the pressure at which will fault the machine if reached during the machining cycle. If the system recharges more then lets say 3 times in a minute, the system alarms out due to excessive cycling which can all be set up in the PLC. Unclamp and clamp circuits can both work under this concept though a recharge is not often needed for the unclamp. Single actuating and double actuating fixtures can be different to save costs, but this type of design setup should work for just about any fixture you would ever want to put on it.

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)

  1. Directional control valves- Poppet (Least expensive,preferred for low flow small systems) or spool type (can handle more contaminants in oil)

  2. P.O. check valves on A+B

  3. Tapping Block for pressure switches on A and B

  4. Pressure switches on A+B (Sometimes only A port I'll explain later)

  5. Dump valve

  6. Pressure reducing valve (not usually needed)

  7. 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.