Hydraulic Cylinder Cushions Can Prevent Mechanical Shock

A racecar on a track can stop one of two ways. If the driver applies the brakes, the vehicle slows in a controlled and gradual way. If the car hits the wall, the change in speed is abrupt and violent. The car will likely sustain mechanical damage as it absorbs the change in energy.

In applications involving high inertia loads, a sudden stop at the end of stroke can cause mechanical shock. The effects can range from a mild annoyance to serious operator risk and equipment damage.

End-of-Stroke Shock Damages Equipment and Creates a Safety Risk

When the load moved by a cylinder comes to a sudden stop, it applies stress to the cylinder. It also stresses the machine’s mechanical structure. Excessive stress on structural components and pressure spikes in hydraulic tubes or hoses can cause premature cylinder failure. Shock also degrades machine productivity by jolting material out of a bucket or container.

Another, even more critical concern is operator safety. “Hard stops can throw equipment operators off aerial platforms, and falling debris can strike them as well. Position-sensing cylinder technology is one way to prevent end-of-stroke shock, but it can be expensive. A more economical option for mobile equipment OEMs is adding cylinder cushioning to your design.

Cylinder Cushioning Options for High Speeds and Heavy Loads

Hydraulic cylinder cushioning slows the speed of a cylinder piston before it reaches the end caps. Decelerating the cylinder rod near end-of-stroke stops the piston from striking the hydraulic cylinder ends, preventing mechanical shock. Additional benefits include:

• Reduced noise and vibration
• Improved performance when moving heavy loads at high speeds
• Less cylinder maintenance and an extended operating life

Cylinder cushioning for hydraulic cylinder ends comes in two options: spear-type cushioning and piston cushioning. We’ll go into detail on the technical differences below.

Welded Hydraulic Cylinders

Pre-Engineered Hydraulic Cylinders

Custom Hydraulic Cylinders

Spear-type Cushioning for Hydraulic Cylinder Ends

Tie-rod construction cylinders commonly use spear-type cylinder cushioning, and welded construction cylinders use it as well. You can find both adjustable non-adjustable spear-type cushioning for heavy loads. This type of cylinder cushioning can be effective but has some shortcomings to consider.

The design has a spear or sleeve that enters and exits a concentric pocket. If the spear and pocket diameter difference is too small, there is a risk of metal-to-metal contact and galling. If the clearance is too large, the effective orifice will be too large, making the cushion ineffective.

A downside from the design standpoint is that the cushioned flow has two parallel paths. Oil flows through the annular area created by the spear and pocket. It then passes across the fixed orifice or the adjustable needle valve. This combination creates a complex scenario for predicting the flow. The spear-type cushioning design also requires extra space in the head and end cap. This extra space provides room for the needle-adjustment valve and the incoming-flow check valve.

Lastly, although the ability to adjust the cushion may have advantages in some circumstances, it also allows for incorrect adjustment. For example, an operator seeking to improve productivity without understanding the potential negative consequences.

Piston Cushioning for Hydraulic Cylinder Ends

Piston cushioning offers advantages over spear-type cushioning for hydraulic cylinder ends. Designers can tailor this solution for a wide range of flows, and it performs especially well at lower flow rates. The controlled flow passes through a single orifice, allowing for more predictable cushion performance. The piston integrates both the orifice and the check valve, which keeps the cylinder compact. Piston cushioning is non-adjustable, preventing malfunctions caused by improper adjustments.

In addition to the typical bidirectional elastomeric piston seal, designers can add a cast-iron piston ring on the blind end to provide a cushion at full retract. They can also install the ring on the rod end for a cushion at full extend, and some designs use rings on both ends. In some designs, it appears on both ends, as shown in figure 1. The groove for the cast-iron ring is wider than normal. This wider groove allows the ring to shift slightly during operation. There is also a series of axial holes and a single cross-drilled hole on each end of the piston.

These features do not affect the cylinder operation during most of the stroke. Flow can freely enter and exit the cylinder, and the elastomeric piston seal prevents internal leakage. The cast-iron ring can “float” in the wide groove, and the pressure is the same on both sides of the ring. The cast-iron ring can ‘float’ in the wide groove because the pressure is the same on both sides of the ring. When the ring passes the port, the exiting flow moves through the axial and cross-drilled holes. This movement creates a pressure drop. With higher pressure on one side, the ring shifts to the opposite side of the groove. The flow then passes through the single cross-drilled hole.

If properly sized, the orifice controls the flow rate of oil exiting the cylinder. The pressure of the incoming fluid increases until it reaches the maximum level determined by a component outside the cylinder, commonly a relief valve or a controller on a variable displacement pump. When the pressure reaches this limit, the relief valve opens and diverts the flow that cannot enter the cylinder. If it does not open, the pump displacement must decrease to reduce the flow (figure 2).

Although the cushion helps slow the cylinder at end-of-stroke, the cylinder must operate immediately at normal speed when the direction reverses. Although the cushion helps slow the cylinder at end-of-stroke, the cylinder must operate immediately at normal speed when the direction reverses. This is why the groove for the cast-iron ring is wider than normal. Just as the pressure drop of the exiting flow moved the ring to one side of the groove, the pressure drop of the entering flow now moves it to the opposite side. The flow can then pass under the cast-iron ring and out the axial holes with minimal or no restriction. This feature produces a fast start-up. After the cast-iron ring passes the port, fluid flows directly into the cylinder, and once again, the cushion features do not affect performance until the next time it reaches the end-of-stroke (figure 3).

The Effects of Pressure on Cylinder Cushioning

When working with cylinders with any cushion, it is vital to consider the possibility of pressure intensification. To reduce the cylinder speed, the exiting flow must be restricted so that the incoming flow reaches the maximum pressure. However, because of the area difference on each piston side, the exiting flow will not be the same pressure as the incoming flow.

On the extend side, also called the blind or cap end, the fluid under pressure acts on the full-bore diameter of the cylinder. On the retract or rod side, the fluid does not act on the center area because of the rod. It only acts on the annular area between the rod and the bore. The ratio of the extend area to the retract area is known as the cylinder ratio. This ratio is typically in the range of 2:1 to 3:1, but it can be as high as 10:1 if the rod is large relative to the bore (figure 4).

If the pressure is controlled by a main system relief set at 3,000 psi (207 bar) and the cylinder has a ratio of 2:1, when the cylinder is retracting and the cushion is active, the rod side pressure increases to 3,000 psi (207 bar). Because the extend side area is greater by a factor of two, the resulting pressure on the extend side is calculated by dividing by two, or 1,500 psi (103 bar).

This pressure is used to design the control orifice size in the piston for the designed flow rate. If the same cylinder has a cushion at full extend and the extend-side pressure increases to 3,000 psi (207 bar), the resulting pressure on the rod side becomes 6,000 psi (414 bar). This higher pressure is completely contained within the cylinder. It is not measured with a cylinder port gauge, nor can it be prevented or limited with an external relief valve.

In addition to using this pressure to determine the orifice diameter, it must also be considered when selecting seals, tube-wall thickness, and head-retention methods. These factors help prevent cylinder failure. If the cylinder has a large-diameter rod and therefore a high cylinder ratio, design challenges increase. It may not be economically feasible to design the cylinder for the resulting rod-side pressure, even at low system pressures.

It may be necessary to add a relief valve set at a lower pressure specifically for the extend side of the cylinder. If that setup does not provide enough extend-force, the cylinder may not be a good candidate for a cushion at full extend. In that case, deceleration should be handled by a different method.

Removing the potential for incorrect adjustment can be a benefit in some applications. In others, however, the benefits of adjustment outweigh the risk. It is also possible to adapt the cushion-piston design for these applications. The end cap or head gland can be modified to accommodate an adjustment valve.