Hydraulic jack refers to a jack that uses a plunger or a vérin hydraulique as a rigid jack. It has the characteristics of compact structure, stable work, strong top support and self-locking. The hydraulic jack has strong supporting capacity, and the supporting force of heavy hydraulic jacks exceeds 100t.

Working principle of hydraulic jacks

1. Pump suction process

When lever handle 1 is lifted by hand, the small piston is driven upward, and the sealing working volume in the pump body 2 increases. At this time, since the oil discharge check valve 3 and the oil discharge valve 8 respectively close the oil paths where they are located, the working volume in the pump body 2 increases to form a partial vacuum. Under the action of atmospheric pressure, the oil in the oil tank opens the oil suction check valve 4 through the oil pipe and flows into the pump body 2 to complete an oil suction action.

2. Pumping oil and heavy lifting process

When lever handle 1 is pressed down, the small piston is driven down, the working volume of the small oil cavity in the pump body 2 is reduced, the oil in it is squeezed out, and the oil discharge check valve 3 is pushed open (at this time, the oil suction one-way Valve 4 automatically closes the oil circuit to the oil tank), and the oil enters the hydraulic cylinder (oil chamber) 11 through the oil pipe. Since the hydraulic cylinder (oil chamber) 11 is also a sealed working volume, the force generated by the incoming oil due to being squeezed will push the large piston up and lift the heavy object to do work. Repeatedly lifting and pressing the lever handle can make the heavy object rise continuously and achieve the purpose of lifting.

3. Heavy object falling process

When the large piston needs to return downward, open the oil drain valve 8 (rotate 90°), then under the action of the weight of the heavy object, the oil in the hydraulic cylinder (oil chamber) 11 flows back to the oil tank 5, and the large piston descends to in situ.
Through the working process of the hydraulic jack, we can conclude that the working principle of hydraulic transmission is: using oil as the working medium, the movement is transmitted through the change of the sealing volume, and the power is transmitted through the internal pressure of the oil. Hydraulic transmission is essentially an energy conversion device.

Classification of hydraulic cylinder

Hydraulic jacks are divided into: general hydraulic cylinders and special hydraulic cylinders.

Universal hydraulic jacks are suitable for various lifting operations with small lifting heights. It consists of an oil chamber, oil pump, oil storage chamber, piston, crank handle, oil valve and other main parts. When working, as long as the crank handle is moved back and forth, the manual oil pump continuously presses oil into the oil cylinder. Due to the continuous increase of the oil pressure in the oil cylinder, the piston and the weight on the piston are forced to move upward together. Open the oil return valve, and the high-pressure oil in the oil cylinder will flow back to the oil storage chamber, so the weight and the piston will fall together.

The special hydraulic jack is a special tensioning machine. When making prestressed concrete elements, tension is applied to the prestressed steel bars. Dedicated hydraulic jacks are mostly double-acting. There are two commonly used types: through-heart type and cone-anchor type.

Feed-through jacks are suitable for tensioning tendons or wire bundles. It is mainly composed of a tension cylinder, top pressure cylinder, top pressure piston and spring. Its characteristics are: there is a piercing hole along the axis of the stretching machine, and the steel bar (or steel wire) is anchored by the tool at the tail after being penetrated.

When tensioning, open the front and rear oil nozzles, supply oil from the rear oil nozzles to the tensioning working oil chamber, and the tensioning cylinder block moves backward. Since the cylinder cable is anchored on the tool anchor at the top of the jack, the jack stretches the cable through the tool. When the cable is stretched to the desired length, close the rear grease nipple. The oil is fed from the front oil nipple into the top pressure cylinder so that the top pressure cylinder piston is moved forward to withstand the anchor plug, and the anchor plug is pressed into the anchor ring, thereby anchoring the wire rope. Open the rear oil nozzle and continue to feed oil from the front oil nozzle. At this time, the tension cylinder moves forward and the oil in the cylinder returns. Finally, open the front oil nozzle to make the oil in the top pressure cylinder return, and the top pressure piston returns to its original position due to the action of the return spring.

How to use hydraulic jack?

1) Before use, it is necessary to check whether all parts are normal. Mainly check the piston, joint, high-pressure hose, etc. for oil leakage.

2) Strictly abide by the regulations in the main parameters during use, and avoid super high overload. Otherwise, when the lifting height or lifting tonnage exceeds the specified value, serious oil leakage will occur at the top of the hydraulic cylinder.

3) If the amount of oil in the manual hydraulic pump is insufficient, it is necessary to add fully filtered hydraulic oil to the pump before it can work.

4) The choice of the center of gravity of the heavy object should be moderate. Reasonably choose the focus point of the jack, the bottom surface should be flat, and at the same time, the soft and hard conditions of the ground should be considered, whether it is necessary to pad tough wood, and whether the placement is stable, so as to avoid the load sinking or tilting.

5) After the jack lifts the heavy object, the support should be used to support the heavy object in time.

6) If several jacks need to be lifted at the same time, in addition to the correct placement of the jacks, it should also be noted that the load of each jack should be balanced. Pay attention to keeping the lifting speed synchronized to prevent the weighted object from tilting and causing danger.

7) When using the jack, first place the hydraulic cylinder, and then tighten the oil drain screw on the hydraulic pump to work. To make the piston rod descend, loosen the handwheel of the manual hydraulic pump slightly counterclockwise, the hydraulic cylinder is unloaded, and the piston rod gradually descends. Otherwise, it will be dangerous to descend too fast.

8) The detachable jack adopts a spring return structure, which can be quickly taken out after lifting, but the jack cannot be pulled by the connected hose.

9) Due to the small lifting stroke of the jack, the rated stroke should not be exceeded during use to avoid damage to the jack.

10) During the use process, the violent vibration of the jack should be avoided (for example, when the jack is loaded, hit the workpiece with a hammer)

Precautions for hydraulic jacks

1) If there is a runaway phenomenon during use, you can first loosen the oil drain screw on the pump body, hit the vertical head of the pump body down a few times, and then tighten the oil drain screw, you can continue to use.

2) When using, do not add partial load or overload, so as to avoid the danger of jack damage. When there is a load, do not remove the quick connector to avoid accidents and damage to the parts.

3) Hydraulic jacks use oil as the medium, and maintenance of the oil and equipment must be done well to avoid blockage or oil leakage, which will affect the use effect.

4) For a new or long-established hydraulic jack, because there is more air in the hydraulic cylinder, the piston rod may jump slightly when it is first used. The hydraulic jack can be reciprocated 2 to 3 times with no load to remove the air in the cavity. Hydraulic jacks that have been idle for a long time will harden due to the long-term non-working of the seals, thus affecting the service life of the hydraulic jacks. Therefore, when the hydraulic jacks are not in use, they should be reciprocated with no load 2 to 3 times a month.

Maintenance of hydraulic jacks

(1) When the hydraulic jack is used, a flat, tough and oil-free wooden board should be placed at the bottom to expand the bearing surface and ensure safety. Do not use iron plates instead of wooden boards to prevent slippage.

(2) It is required to be stable when lifting. After the weight is lifted a little, check whether there is any abnormal situation. If there is no abnormal situation, the lifting operation can be continued. Do not lengthen the handle arbitrarily or operate it too hard.

(3) Do not overload, do not exceed the rated height. When a red line appears on the sleeve, it indicates that the rated height has been reached and the jacking should be stopped.

(4) When several hydraulic jacks are working at the same time, they should be directed by a special person to make the lifting or lowering synchronously. Wooden blocks should be supported between two adjacent hydraulic jacks to ensure the interval prevent sliding.

(5) When using the hydraulic jack, always pay attention to the sealing part and the pipe joint part, and they must be safe and reliable.

(6) Hydraulic jacks are not suitable for places with acid, alkali, or corrosive gas.