• Hy Power Clamps Private Limited

Hydropneumatic Automation

Hydropneumatic clamping automation can be termed as low cost automation as compared to clamping automation, using a power pack. A hydropneumatic intensifier gives hydraulic leverage similar to mechanical leverage. The hydropneumatic intensifier is widely used on conventional machines as well as SPMs, where a hydraulic power unit is not available on the machine. Low cost, simpler circuit and “no oil heating” as compared to hydraulic power unit, are some of its advantages. Another consideration for selecting hydropneumatic automation is production volume. Manual clamping is used in prototype production or batch production where production volumes are low. As the production increases, even conventional machines are dedicated for specific operation. Hydropneumatic clamping is generally used at this production level (50 to 200 jobs per shift).

Principle of intensifier: In the static condition, (Pressure x Area) at airside is equal to (Pressure x Area) at oil side. The hydropneumatic intensifier has one oil cylinder and one air cylinder. The pistons of both these cylinders are connected to each other. In single acting intensifier, the air cylinder is single acting, spring return. The air piston is the driving piston and the oil piston is the driven piston.

Operation: When air is allowed in the intensifier by a DC valve, oil on the hydraulic side gets pressurised and is forced out. The oil output operates the clamping cylinders and the job gets clamped. After releasing the air pressure, due to the spring in the intensifier, the air piston returns back and the job gets de-clamped. Oil is pushed back into the intensifier due the spring in the cylinder.

Leakage compensation (make up oil system): This is an important feature of hydropneumatic intensifier and it is very similar to automobile brake oil compensation. The oil-side of the hydropneumatic intensifier, hydraulic hoses and cylinders form a close loop system. Oil in this loop is a confined fluid. There must be a leakage compensation for the confined oil, if there is any leakage (across piston of cylinder and through connectors). The make-up oil system compensates for leakage by adding small volume of oil at every stroke. Any leaked-out volume causes vacuum in unclamp mode and is automatically compensated due to the atmospheric pressure.

Selection of intensifier: Intensification ratio can be selected as air pressure available. For 5 to 7 bar air pressure, select an intensification ratio of 28 so as to get about 150 bar hydraulic output. For 3.5 to 4 bar air pressure, select an intensification ratio of 40 to get about 150 bar hydraulic output. Oil volume for every cylinder is given in the catalogue. Sum up all the oil volumes of the cylinders to get the total oil requirement. Intensifier output must be more than the total oil volume required.

Type of cylinders: Single acting, spring return cylinders only can be used with hydropneumatic intensifier. Thumb rule for cylinder force selection is given by bolt size, e.g., for M12 – 10 kN, M16 – 20 kN and M20 – 30 kN.

Points to be remembered while designing the system: Keep the system as simple as possible. Keep minimum number of connections in the circuit to avoid possibility of leakage. Try to minimise the hose length and use a quick exhaust valve to get a quick return stroke. Use bonded seals for leak proof connections. Keep the intensifier preferably at the highest points for easy bleeding. In case of a rigid piping, use a slant piping going towards intensifier for easy bleeding. For easier bleeding, avoid loops in hoses or rigid piping.

Negative points against Hydropneumatic LCA are that operator must be aware of function of make-up oil system and bleeding procedure. As spring return cylinders are provided with breather, the cylinder can get damaged due to flood coolant.