August 17, 2021

Selecting the Right Telescopic Hydraulic Cylinder

Selecting the Right Telescopic Hydraulic Cylinder

When you need the power of a hydraulic cylinder, but you don’t have space for a traditional rod cylinder, the best solution is a telescopic hydraulic cylinder. Able to lift just as much as a standard cylinder, telescopic models can fit the same amount of power while only taking up 20% to 40% of its extended length while collapsed.

There are several applications where a telescopic hydraulic cylinder is useful and the only way to perform certain functions.

This article will look at some of the different types of telescopic hydraulic cylinders and how they function. We’ll also look at some of the most common applications of these telescopic hydraulic cylinders and how you can make sure you choose the right cylinder for your needs.

How Telescopic Hydraulic Cylinders Work

Tank truck with a telescopic hydraulic cylinder

While there are several varieties of telescopic hydraulic cylinders, they all work on the same principle. These cylinders are constructed from a series of steel tubes, which are nested in descending size.

The largest pipe, known as the barrel, is visible when the cylinder is retracted. The smallest pipe, or plunger, contains the end that attaches to the working load. The other sections, which extend to make up the rest of the stroke, are known as stages.

For stability reasons, the maximum number of cylinders in a telescopic hydraulic cylinder tops out at six. Any more than that, and the system tends to become unstable, presenting multiple problems at both the design and execution stages.

Hydraulic oil pumps into the system to extend the device. The oil pushes the stages out from biggest to smallest. Each stage extends to its full length before the next stage starts to extend until, finally, the plunger is extended all the way. As the cylinder retracts, the process repeats itself in reverse — the plunger retracts before the following stage retreats, and so on.

Bearings and Seals

Every stage in a telescopic hydraulic cylinder is supported by guide bearings, which keep the cylinders adequately aligned. These bearings define how far the inner stage projects from the previous stage and how much the two stages overlap.

There are a wide variety of options available when it comes to seals, each with a slightly different design and utility purpose. For example, two standard designs — multi-lip seals and hinged-V seals — are installed using stop rings and packing nuts, and these fit inside each telescoping section. The seal is formed between the outer diameter of the smaller stage and the inner diameter of the stage housing it. These seals create the pressure that allows the telescopic hydraulic cylinder to extend.

The Types of Telescopic Hydraulic Cylinders

The two primary types of telescopic hydraulic cylinders are single-acting and double-acting — most variations arise from combining these two styles.

Single-Acting Telescopic Hydraulic Cylinders

A single-acting cylinder requires pressure to extend. However, rather than retracting under pressure, the device relies on external forces to retract. This is most commonly the case when a constant load, such as gravity, is working against the cylinder, which allows the cylinder to retract. 

While a single-acting cylinder requires hydraulic oil to force each stage out of the cylinder, the retraction process occurs when that pressure is removed and the weight working against the cylinder gradually overcomes the hydraulic pressure. This forces the hydraulic oil back into the reservoir.

Double-Acting Telescoping Hydraulic Cylinders

Excavator with double-action hydraulic cylinders

As you might expect, a double-acting cylinder uses pressurized hydraulic fluid to extend and retract its stages, making them ideal for situations where gravity or another external force can’t be relied on to retract the cylinder.

In general, extending these cylinders works exactly the same as in a single-acting cylinder. That being said, to use pressure to retract these cylinders, each stage that moves has oil transfer holes built into it. As each stage retracts, it exposes the oil transfer hole in the next stage until the entire cylinder has retracted into the barrel.

This design creates complications that must be considered. Most double-acting telescopic hydraulic cylinders have both extension and retraction ports located in the plunger. But to protect the hoses and ports and limit the amount of hosing required to run the cylinder, the plunger remains anchored to the base, with the barrel located at the working end of the load.

Typical Applications for Telescopic Hydraulic Cylinders

There are several situations in which a telescopic hydraulic cylinder is the right tool for the job. For instance, single-acting cylinders are extremely common on dump trucks, and for a good reason. 

The container must be tilted to somewhere between 45 and 60 degrees to empty a dump truck. Regardless of the length of the container, the required stroke length to pivot the container on its hinge would be nearly the length of the truck bed. There would be no way to retract the rod all the way, making it impossible to load the truck or even drive it.

With telescopic hydraulic cylinders, on the other hand, the retracted cylinder takes up far less space. And with the weight of the container, it doesn’t require a double-acting cylinder to retract and bring the bed back to a horizontal position.

On the other hand, you’re more likely to see a double-acting cylinder when the hydraulic force is being used horizontally, meaning it can’t rely on gravity. Consider sanitation vehicles, where refuse is compacted throughout the route. The compactor is operated by a double-acting telescopic hydraulic cylinder, allowing the platen to press against the trash, then retract to allow more refuse to be added. Because the platen is operated from above, it requires a double-acting cylinder to pull the platen back to the top of the truck.

Other situations require telescopic cylinders, as well. For example, constant thrust and constant speed cylinders are used for deep drilling in which every stage is extended simultaneously under constant pressure. However, while you may find specialized telescopic hydraulic cylinders like this, it’s far more common to encounter single- and double-acting cylinders.

How to Choose the Right Telescoping Hydraulic Cylinder

hydraulic cylinder

Finding the right telescoping hydraulic cylinder is a matter of knowing how long the stroke needs to be and how much pressure you’ll need to handle the maximum load of your equipment. That requires understanding a few pieces of information about your specific needs and performing the proper calculations.

Let’s use a dump truck cylinder as an example. There are two measurements we need right off the bat. First, you need to identify where the container is hinged to the truck frame. Then, you need to measure the distance (in inches) between that hinge and the center of the load being lifted. This will be the middle of the dump body most of the time, but that may not always be the case.

The second measurement (also in inches) starts at the hinge again, but this time measures to where the cylinder connects to the dump body. This number will typically be much longer than the first.

Force and Stroke Calculations

The next step is to determine the initial amount of force you need to lift the load. The most difficult part of the lift is right at the start, with less force required as the angle of the dump body increases.

To figure out the force needed to lift the load, multiply the anticipated load in pounds (including the weight of the dump body) by the first measurement you took. Then, divide that number by the second measurement, which will give you the proper force you need to lift.

For example, let’s say you want to lift 14,000 pounds, with a length of 80” to the center of the load and a distance of 160” between the hinge and the cylinder base point.

14,000 x 80 = 1,120,000 / 140 = 8,000

That means that you need to be able to produce 8,000 pounds of force to start lifting your load of 14,000 pounds.

To determine your stroke length, you simply multiply the distance between the hinge and the cylinder base by a multiplier based on the dump angle. The dump angle is the angle created between the truck frame and the dump body as the cylinder extends. The angle you need is simply the angle at which the dump body can be emptied completely.

Dump AngleMultiplier

If you’re lifting your bed to 49˚, you would simply multiply the distance between hinge and cylinder base (140”) by the multiplier (.830) to get a stroke length of 116.2”.

See the MAC Hydraulics Difference

Whether you need a new piece of hydraulic equipment, maintenance, or even repairs, MAC Hydraulics is the answer. Our technicians have hydraulic experience across a variety of industries, including construction, waste management, and more. Contact us today for more information about telescopic hydraulic cylinders and all of your other hydraulic equipment needs.

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