BIZOMAT MAKEOS · Hydraulics
Cylinder Cushion Check (Kinetic Energy, Deceleration, Cushion Pressure, Largest Speed)
When a hydraulic cylinder decelerates a moving load in its end cushion, how high does the trapped cushion pressure rise, and what is the fastest speed the cushion can stop it from?
Check of the end cushion of a hydraulic cylinder stopping a moving load at the rod end: the kinetic energy and the deceleration over the cushion length (as the uniform acceleration tool), the force needed to stop the mass plus the drive force still pushing on the full bore and any external load, the pressure in the trapped annulus that this needs, and the highest approach speed the cushion can take at its rated pressure, against the cushion pressure rating.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Moving mass | kg | Mass of everything moving with the rod, including the rod and piston. |
| Speed entering the cushion | m/s | Speed of the piston as it enters the cushion. |
| Cushion length | mm | Length of the cushion spear or sleeve. |
| External force pushing the same way | N | Other force in the direction of motion during cushioning, such as gravity on a falling load (negative if it opposes). |
| Cylinder | ||
| Bore | mm | Cylinder bore. |
| Rod diameter | mm | Rod diameter (the cushion traps oil in the annulus on extension). |
| Pressure still on the full bore during cushioning | bar | Pressure the pump keeps on the full bore while the cushion acts (relief setting for the worst case). |
| Cushion pressure rating | bar | Highest pressure the cylinder maker allows in the cushion. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Moving mass
- What it is
- Everything the cylinder has to bring to rest in the cushion: the piston and rod plus the slide, tooling, workpiece and anything else moving with them.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0.000001 kg to 1000000 kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 200 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The force to decelerate the mass, and its share of the cushion pressure, rise in proportion to it.
- Get it right
- Include the piston and rod as well as the load; weigh the parts or take masses from the drawings or CAD model.
What this means: Speed entering the cushion
- What it is
- How fast the piston is moving at the moment the cushion spear or sleeve closes off the free oil path, after any speed reduction before the cushion.
- Units and format
- Enter a number in m/s: metres per second. The form accepts 0.000001 m/s to 20 m/s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.5 m/s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Deceleration and kinetic energy rise with the square of this speed.
- Get it right
- Use the speed at the cushion entry, not the average stroke speed; a cylinder often runs fastest just before the end.
What this means: Cushion length
- What it is
- The distance over which the cushion acts: the length of the spear or sleeve that enters the cushion bore and traps oil before the piston reaches the end cap.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A longer cushion lowers the deceleration and the cushion pressure, and raises the largest safe speed.
- Get it right
- Take it from the cylinder maker’s data; it is the cushion length, not the full stroke of the cylinder.
What this means: External force pushing the same way
- What it is
- Any other force acting in the direction of motion while the cushion stops the load, such as the weight of a load being lowered or a process force pushing the same way.
- Units and format
- Enter a number in N: newtons. The form accepts -10000000 N to 10000000 N. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- It adds directly to the stopping force, so a positive value raises the cushion pressure and lowers the largest safe speed.
- Get it right
- Enter a force that resists the motion as a negative number; 0 means no external force.
Cylinder
What this means: Bore
- What it is
- The inside diameter of the cylinder barrel, which sets the full piston area the drive pressure pushes on and, with the rod, the trapped annulus area.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 2000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 63 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger bore increases the drive force on the full bore but also the annulus area that resists it.
- Get it right
- Use the nominal bore from the cylinder nameplate or data sheet, not the outside diameter of the tube.
What this means: Rod diameter
- What it is
- The diameter of the piston rod; the cushion traps oil in the ring between the rod and the bore when the cylinder extends.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 2000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 36 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger rod shrinks the annulus area, so the cushion pressure rises.
- Get it right
- It must be smaller than the bore; a thicker rod leaves a smaller annulus to take the stopping force.
What this means: Pressure still on the full bore during cushioni…
- What it is
- The pressure the pump or circuit still holds on the full-bore side of the piston while the cushion is acting, which keeps pushing the load into the cushion.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0 bar to 1000 bar. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- It adds drive force on the full bore, raising the cushion pressure and lowering the largest safe speed.
- Get it right
- For the worst case use the relief valve setting, not the working pressure; enter 0 only if the supply is cut before the cushion.
What this means: Cushion pressure rating
- What it is
- The highest pressure the cylinder maker allows in the cushion chamber, used as the pass or fail limit and to find the fastest speed the cushion can take.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0.000001 bar to 2000 bar. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 250 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the pass or fail line and the largest safe speed; the calculated cushion pressure does not change.
- Get it right
- Ask the cylinder maker for the cushion rating; it is not necessarily the same as the nominal working pressure of the cylinder.
Method and formulas
- Deceleration a = v² ÷ 2s over the cushion length (as the uniform acceleration tool); kinetic energy ½ m v².
- Stopping force = m a + drive pressure × bore area + external force; cushion pressure = force ÷ annulus area.
- Largest speed = √(2s (rating × annulus − drive force − external force) ÷ m).
- Example: 25 J stopped in 20 mm raises the cushion to 154.44 bar; up to 2.0645 m/s is safe.
Limitations
- Uniform deceleration over the cushion; real cushions peak at the start unless they are profiled.
- Rigid oil; oil compressibility and hose swell soften the peak a little.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a cylinder stopping a heavy slide
- Find the fastest safe speed into the cushion
- Decide whether external shock absorbers are needed
Who it is for
- Hydraulic engineers
- Machine designers
- Maintenance engineers
What you get
- Kinetic energy and deceleration
- Force to stop and cushion pressure
- Largest speed for the cushion rating
Interpreting the result
- Pump pressure on the full bore often adds more cushion pressure than the load itself: dropping the pressure during cushioning helps most.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 175–181, 281–286. Topic reference; no source tables or recommended settings are reproduced.
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