Rexroth CD vs CG Series: Cushion vs Non-Cushion Hydraulic Cylinders

Rexroth CD vs CG Series: Cushion vs Non-Cushion Hydraulic Cylinders

The "D" and "G" in Rexroth cylinder model codes look like a single-letter difference. In application terms it's actually one of the bigger choices you make in a cylinder spec — whether the cylinder includes end-of-stroke cushioning or not. Get this wrong on a press, a stamping line, or any application with momentum at end of travel, and you trade off either machine life or system noise. Get it right and a cylinder will run for years without the shock-induced fatigue that retires its less-suitable siblings early.

1. What CD and CG Actually Mean

In Rexroth model coding:

• CD — Cylinder, Double-acting, without end-of-stroke cushioning
• CG — Cylinder, with end-of-stroke cushioning (Gedämpft in German)

Beyond that single letter, the two share most of the rest of the spec: bore range, rod diameter, mounting options, pressure rating, and seal package. A CDH1 and a CGH1 in the same size are built around the same body and seals — the difference is internal hardware in the end caps that decelerates the piston as it approaches end of stroke.

The rest of the model code tells you the heavy/medium/light series and pressure class:

So a CDH1 is a heavy-duty 350 bar cylinder without cushion. A CGH2 is a heavy-duty 250 bar cylinder with cushion. A CDL1 is the compact light series with no cushion — typically 25 mm to 200 mm bore, 14 mm to 125 mm rod diameter, double-acting single-rod construction. A CGL1 would be the same compact series with cushion added.

2. How Cushioning Works Inside the Cylinder

End-of-stroke cushioning is a hydraulic deceleration mechanism, not a mechanical bumper. The principle is straightforward:

As the piston approaches end of stroke, a cushioning sleeve or pin on the piston enters a matching bore in the end cap. This effectively closes off the main return port. The remaining oil between the piston and the end cap can only escape through a small adjustable throttle (or a fixed throttle in fixed-cushion variants).

That restriction creates back-pressure. Back-pressure slows the piston. Over the last 15–30 mm of stroke, the piston decelerates smoothly to near-zero velocity at the end position, rather than slamming into the end cap.

Most CG cylinders offer adjustable cushioning via a screw on the end cap — tighter restriction for higher-mass loads or higher velocities, lighter restriction where the deceleration would otherwise overshoot. A few smaller series use fixed cushioning factory-set for typical loads.

3. When Cushioning Matters — and When It Doesn't

Cushioning earns its place when the cylinder is moving with enough kinetic energy at end of stroke that an uncontrolled stop would cause damage or noise. The kinetic energy depends on three things: load mass, velocity at end of stroke, and how much travel the cushion has to dissipate it across.

Cushioning is critical when:

• The cylinder moves a heavy load — typically above 50 kg per cylinder, scaling up sharply with bore size
• Stroke velocity at end of travel exceeds about 0.1 m/s
• The load decelerates without external mechanical brakes or limit-valve circuits
• The application demands quiet operation (food packaging, hospital equipment, residential building lifts)
• Frequent cycling exposes the cylinder to repeated impact stress (presses, stamping, conveying)

Cushioning is not needed when:

• Loads are light enough that residual momentum at end of stroke is negligible
• The cylinder moves slowly throughout the stroke (under 0.05 m/s, e.g. clamping, holding)
• External deceleration is built into the hydraulic circuit (deceleration valves, proportional control)
• The cylinder is purchased as a generic clamp where end-position accuracy isn't critical
• Cost sensitivity rules out the cushion premium for low-volume mass-produced equipment

A rough field guideline: anything above 100 kg working load moving faster than 0.2 m/s should have cushioning unless circuit-level deceleration is already in place. Below that, the choice is application-driven.

4. Performance and Lifespan Implications

The visible effect of running a CD cylinder where a CG belongs shows up over months and years, not days.

Without cushioning (CD), high-velocity end stops produce:

• Mechanical shock transmitted to mounting bracket, machine frame, and supporting bearings
• Hydraulic shock pulses (water-hammer in the return line)
• Stress cycling on the piston-rod weld and the cylinder head bolts
• Audible "clunk" at end of travel — noise compliance issues in some environments
• Accelerated wear on the piston seal at the end-of-stroke contact area

With cushioning (CG), the same load decelerates smoothly:

• Mechanical and hydraulic shocks reduced by 70–90% on properly-tuned cushions
• End-cap bolts and structural welds stay within fatigue limits
• Seal life extended (the seal isn't subjected to slam stops)
• Noise drops substantially — typically 5–10 dB measured at the cylinder
• Cycle time can sometimes be increased without damage because end-of-stroke energy is managed

The trade-off is cost. A CG cylinder typically costs 15–25% more than the equivalent CD. For applications where it's actually needed, that's recovered many times over in extended seal-change intervals and reduced structural maintenance.

5. Series Overview Across the Rexroth Lineup

The CDL1 is one of the most widely deployed in the lineup because it covers a useful range without the cost and weight of the heavy-duty series. Standard build is single-rod double-acting, 25 mm to 200 mm bore, rod diameters 14 mm to 125 mm. Multiple mounting variants are available: MT4 (trunnion), MF3/MF4 (flange), MP (clevis), and so on. Optional dust-protection, low-friction seals, and Industry 4.0 sensor integration are configurable through suffix codes (C1X, B1CH, and others).

6. How to Choose

A practical sequence:

Step 1 — Calculate end-of-stroke kinetic energy. E = ½ × m × v² where m is the moving mass and v is the end-of-stroke velocity. Above roughly 50 J per cycle, cushioning becomes important. Above 500 J, it's essentially mandatory for any reasonable service life.

Step 2 — Check whether the hydraulic circuit handles deceleration externally. Proportional valves, deceleration valves, or PLC-controlled velocity ramps can manage end-of-stroke without an in-cylinder cushion. If they're already in the design, a CD may be sufficient.

Step 3 — Consider cycle frequency. A cylinder cycling 100,000+ strokes per year benefits dramatically from cushioning even at moderate energy. Low-cycle applications (a few hundred per year) can tolerate harder stops.

Step 4 — Check noise constraints. Operating environments with hearing-protection requirements or proximity to public spaces benefit from cushioning regardless of energy levels.

Step 5 — Confirm budget margin. The 15–25% premium for CG over CD is small relative to the cost of premature seal replacement or structural fatigue repair. Don't shortcut a high-energy application to save the upfront cost.

7. Quick Selection Matrix

FAQ

Q: Can I add cushioning to a CD cylinder later by adding parts?

No. The cushion bore in the end cap is part of the cylinder construction, not an aftermarket add-on. Retrofit requires replacing the cylinder or rebuilding it with new end caps — usually more expensive than buying a CG from the start.

Q: How much does cushioning add to the cylinder length?

Typically very little. The cushion sleeve sits inside the existing end cap geometry. Overall envelope dimensions are usually identical to the CD equivalent — interchangeable in the same mounting space.

Q: My CG cylinder still slams at end of stroke. What's wrong?

Either the adjustable cushion screw is open too far, or air is in the cylinder (often after seal replacement), or the cushion sleeve is worn. Adjust the cushion screw smaller first; if that doesn't help, bleed the cylinder; if it still slams, the cushion sleeve needs inspection.

Q: Is cushioning effective in both directions of travel?

Yes — properly designed cushions in both end caps decelerate the piston at both ends of stroke. Single-end cushioning exists for special applications but isn't standard.

Q: Does a CG cylinder lose any stroke length compared to a CD?

The usable stroke is identical. The cushion sleeve operates within the last 15–30 mm of stroke, but the full nominal stroke is still available. Output force is also unchanged — cushioning doesn't reduce force capability.