Hydraulic jack: lifting heavy loads with little effort

Hydraulic jacks are technically sophisticated tools that can be used to move even very large loads with minimal muscle power. Compared with traditional scissor or screw jacks, they offer certain advantages, but also some disadvantages.

Hydraulic jacks: the power of hydraulics

The remarkable effect that hydraulics can generate enormous forces is based on the incompressibility (non-compressibility) of liquids. When a hydraulic cylinder retracts in a closed system, an equally sized cylinder elsewhere extends again by the same distance and at the same speed.

However, it becomes interesting when the cylinders have different cross-sections. In this case, the force is always multiplied in proportion to the cylinders’ cross-sectional area. A retracting cylinder with half the diameter of the extending cylinder pushes the extending cylinder out with four times the force.

Although the extending cylinder moves only 1/4 of the distance of the retracting cylinder, the slave cylinder can be fully extended through sufficient repetitions or a sufficiently long retracting stroke of the master cylinder. In this way, it is possible to generate enormous forces with relatively little technical effort. One application of this is the hydraulic jack.

Design and operating principle of a hydraulic jack

A hydraulic jack consists of the following components:

• Small master cylinder with connected hinge
• Lever bar
• Reservoir
• Master cylinder
• Return valve

The lever bar is connected to the hinge. The user then pumps the oil into the reservoir with downward movements. The pressure that builds up there is balanced by the extending slave cylinder. Backflow of the oil is prevented by the closed return valve. For resetting, the return valve is opened. The oil flows into the reservoir and the slave cylinder lowers back to its starting position.

Types of hydraulic jacks

There are two types:

• Bottle jacks are small, compact tools. Due to their small size, they are very suitable for carrying with you. Their advantages are their high lifting capacity, ease of use, and compact external dimensions.
  
  
• Trolley jacks are hydraulic jacks equipped with a linkage mechanism and a chassis. Trolley jacks are very popular in workshops because they are easy to use. They are less suitable for being carried in a vehicle at all times, as they are heavy and very bulky.

Advantages and disadvantages of hydraulic jacks compared with mechanical jacks

A jack is standard equipment in vehicles. At least vehicles that are equipped with a spare wheel always have such a device on board. These jacks operate purely mechanically, either using a screw principle or a screw-scissor principle. They are significantly lighter and, above all, considerably cheaper to manufacture than hydraulic jacks. In addition, purely mechanical jacks do not pose any risk of leakage. However, hydraulic bottle jacks are more powerful, more compact, and easier to use.

Tips for using jacks

It is absolutely necessary to know exactly where the jack must be positioned before using it. An incorrectly positioned jack can severely damage the bodywork. In the worst case, the entire frame can become distorted, resulting in the car being written off as a total economic loss.

It is therefore advisable to familiarise yourself with the correct handling of the jack and the correct jacking point before an emergency occurs, so that safe use is ensured even in the dark or in the rain.