Telescoping Hydraulic Cylinder Components

Inspections And Testing - Airoflex

Hydraulic cylinders are common in most industries today. Although the concept behind them is simple, components aren't as straightforward. Understanding hydraulic components can increase maintenance efficiency, and cylinder performance. Below is a breakdown of the individual components and their function.

Three Main Cylinder Connection


Top Connection


Pin Eye

The pin eye is the topmost piece on a cylinder. It connects to the smallest (or "final") cylinder stage by a thread and O-ring system. The mounting hole of the pin eye is fitted with a bronze bushing and set screws. The set screws prevent the pin from rotating
inside the pin eye. The brass bushing allows for a replaceable bearing surface. Truck dumpers are heavily loaded and over time the pin connection become out of round or ovaled. The brass bushing deforms instead of the steel. This allows replacement of the bushing instead of the entire pin eye part.


A-Frame Bearings

A-frame bearings mount to the structure of the dumper with four (4) bolts. These bearings are bored in matched pairs to ensure proper alignment. They are often stamped with an "A" and "B" mark to notate the matching pair. The top pin will rotate inside the bearing. The pin has grease grooves that match with the grease zerks on the bearings. Each individual bearing has four (4) set screws. Moving the set screws adjusts the alignment of the bearings after installation.


Top Pin

The top pin connects the pin eye to the A-frame bearings. Top pins will have two grease grooves that will align with the grease zerks on the A-frame bearings. Top pins also have two keeper bolts, one on each side. These bolts ensure that the pin does not work its way out of the bearing over time.

Stage Connection


Telescoping cylinders consist of two to six hollow tubes. These tubes (or "stages") nest inside of each other. A traditional rod cylinder can never extend more than twice its retracted length. Telescoping cylinders can extend up to five times their retracted length. Hollow tubes also allow the stages to be larger diameters while minimizing weight. Telescoping cylinders can generate large forces due to their increased surface area.

Main Stage

The main stage consists of a tube and cylinder base welded together. The cylinder's oil ports are on the cylinder base near the pivot. The serial numbers are stamped on a tag around the edge of the cylinder base.


Internal Stage

Internal stages are hollow steel tubes. There are one to five internal stages on each cylinder. Each tube will have external threads on one end, and three raised bands on the other. The two bands made of brass are wear rings. The third ring, made of steel, is called the stop ring. The stop ring contacts the sleeve of the next largest stage and keeps the stage in place at full extension. All internal stages have slots in both brass rings and holes near the top brass ring and stop ring. The slots and holes allow better oil flow inside the cylinder during retraction.


Final Stage

There are three differences between final stages and internal stages.
1) Final stages will have internal threads on the end
2) The bottom of the tube is capped (no oil flow)
3) There are no drain holes on final stages.



Sleeves are large steel rings which thread onto the end of the internal and main stages. The sleeves serve as a base for the packing and glands to mount to. The smallest sleeve will have an air vent. The vent is used to bleed the cylinder of air prior to operation.



Glands are brass rings with external threads. Each gland will have a matching size sleeve that it will mate to. Glands serve two purposes. The first is to retain the packing in the sleeve. The second is to hold the wiper. The wiper is the blue ring shown with the gland. The wiper's purpose is to remove any oil residue and debris that gathers on the
stages while in use. Wipers can be replaced independently of the gland.



Airoflex packing consists of two polymer rings that seal the oil inside the cylinder. The blue ring provides primary sealing and is softer and more pliable. The purple ring provides secondary sealing. It also provides a base to prevent excess deformation of the blue ring. Each size of packing has a white plastic spacer ring. The spacer ring allows the gland to be set at the correct height for optimal sealing.

Foundation Connection


Most cylinders mount on concrete piers or slabs. Baseplates are cast into the concrete and the base bearing mounts on top. The bottom bearing is replaceable and can be shimmed to ensure proper alignment.


Base Bearing

Base bearings attach to the baseplate and create the lower pivot point for the cylinders. All base bearings have two set screws. These set screws keep the bottom pin from rotating in the bearing.


Bottom Pin

The cylinder rotates between the bottom pin and the cylinder base. The bottom pin has one grease zerk which feeds two grease grooves that align with the bronze bushing in the cylinder base.

Learn More

Click to download the cylinder nomenclature and assembly drawing.