Hydraulic Lifting Cylinders

Hydraulic lifting cylinders are vertically installed hydraulic linear motors. They work against gravity and therefore “lift something up.” This is where their name comes from. Typical mechanical lifting cylinders include, for example, jacks.

Characteristics of Hydraulic Lifting Cylinders

The transmission or amplification of hydraulic forces is based on the incompressibility of fluids and on the difference in cross-sectional area between the master and slave cylinders.

Lifting cylinders are fundamentally slave cylinders. This means they receive the hydraulic pressure and amplify it through their larger cross-sectional area compared to the master cylinder (if present).

The simplest hydraulic systems with lifting cylinders are, for example, hydraulic jacks: Here, a master cylinder amplified by a lever presses into a reservoir containing hydraulic oil. The pressure applied there is transmitted to the slave cylinder, i.e., the lifting cylinder. While it extends over a shorter distance than the master cylinder is immersed in the pressure vessel, it applies a higher force during extension. To reset the lifting cylinder, a check valve is opened and the master cylinder is relieved. The hydraulic oil then flows back into the reservoir (the hydraulic tank) and the lifting cylinder returns to its lower end position.

However, systems consisting of master and slave cylinders are only used for manual operation. These include, for example:

• Pallet trucks
• Hydraulic jacks
• Hydraulic-manual lifting systems such as ramps, flaps, gates, etc.

The combination of hydraulic master and slave cylinders without electromechanical assistance is still quite common. Clutches and brakes in vehicles, for example, are operated according to the same principle. Only in this case, the slave cylinder does not “lift” anything, but exerts its force in a horizontal position. Therefore, in these applications, it is not considered a “lifting cylinder.” In larger systems, the manually operated master cylinder is replaced by an electrically driven hydraulic pump.

Hydraulic Lifting Cylinders – From One to Many

The hydraulic pressure in a system is generated by a pump. This can be manually or electrically driven. The pressure generated by the pump is considered a constant.

If all cross-sectional areas of the master and slave cylinders were the same size, the effective force at the actuator would correspond to the output force of the pump. However, through a simple trick, the force acting on the slave cylinder can be amplified many times over. This requires only an enlargement of the cross-sectional area of the slave cylinder. Pressure is force per area.

The immersing master cylinder displaces a certain amount of volume. This volume remains the same during the expansion of the slave cylinder. However, if the cross-sectional area of the slave cylinder is increased, the extension distance of the cylinder decreases. The effective force, however, increases proportionally with the cross-sectional area. This is illustrated by the following example:

A mechanical master cylinder has a cross-sectional area of 4 cm² and is immersed with a force of 100 N. The slave cylinder has a cross-sectional area of 8 cm².

100/4 = X/8
800/4 = X
200 = X It therefore extends with a force of 200 N!

While its extension distance is then only half as large as the retraction distance of the master cylinder, hydraulics is not about speed, but about force. The master cylinder simply needs to be operated accordingly often, or the pump must work long enough until the slave cylinder has extended the desired distance.

The Disadvantage of Lifting Cylinders and How It Can Be Remedied

A disadvantage of lifting cylinders is their long overall length. Like any hydraulic linear motor, the lifting cylinder also consists of a housing and a piston rod moving within it. The piston rod must be fully retracted into the housing in the lower end position. However, since the performance of a lifting cylinder depends not only on its force but also on its working length, a conflict of objectives can arise here:

The desire for a long stroke combined with the most compact design possible when not in use is difficult to implement for simple linear motors.

A solution is offered by so-called “telescopic cylinders.” This is a special form of lifting cylinder. In the telescopic cylinder, the piston rod is not one-piece, but multi-piece in the form of a telescoping structure. The telescopic cylinder thus combines the functions of housing, piston rod, and lifting cylinder in one unit.

When fully retracted, the telescopic cylinder has only a small installation depth. It then has a significantly larger cross-sectional area than a conventional hydraulic linear motor. However, this is often technically much easier to accommodate than a bulky, single-acting lifting cylinder.

Applications of Hydraulic Lifting Cylinders

Lifting cylinders are frequently found in construction vehicles and dump trucks. They lift the boom arms and sticks or the dump bodies for emptying bulk materials. Lifting cylinders are usually single-acting. This means their return is accomplished by the gravity of the applied load or tool. This makes lifting cylinders more cost-effective than double-acting cylinders.

In addition to manual lifting cylinders, hydraulic lifting cylinders are used in many applications. Typical use cases include:

• Lifting platforms
• Hydraulic presses
• Gates and doors
• Ramps

When the lifting cylinder is combined with a joint, it can be used—especially in telescopic design—in dump trucks.

Additional Variants of Lifting Cylinders

A lifting cylinder does not necessarily have to be hydraulically operated. Depending on the application, pneumatic or electromagnetic or electromechanical lifting cylinders are also in use. However, hydraulic lifting cylinders are the best option in many applications due to their high force.