Knowledge

How to Size an Unclamping Booster: Boost Ratio and Oil Volume, With a Worked Example

2026/ 09/ 22

Choosing an unclamping booster for a spindle comes down to two numbers: the boost ratio, which sets the output oil pressure, and the oil volume, which decides whether the unclamping cylinder completes its full stroke. This article works through both with a real example.

1. First: the booster only unclamps

A BPF/BPFQ uses compressed air on a large piston to drive a small plunger that delivers hydraulic oil:

Output oil pressure (kgf/cm²) = air pressure (kgf/cm²) × boost ratio

The oil drives the spindle's hydraulic unclamping cylinder to push the drawbar and release the tool. Clamping force comes from the spindle's disc springs; the booster plays no part in clamping. It only works during tool change, and uses no air and generates no heat on standby.

2. What to have ready

Working air pressure, with supply tolerance within ±0.1 MPa (0.6 MPa recommended)
Spindle unclamp stroke (mm) and drawbar force (kgf), allowing 20–30% extra on total force
Piston area and stroke of the hydraulic unclamping cylinder (or its total oil volume)
Tool taper: BT30, BT40, BT50
Solenoid voltage: DC24V / AC110V / AC220V

3. Step 1: boost ratio

Required oil pressure ÷ working air pressure = minimum ratio. Choose the next standard ratio at or above it. Common ratios are 6.25, 8, 11, 12.7, 16 and 25:1, up to 51:1.

If the required oil pressure is low, reduce the air pressure with the regulator rather than choosing an oversized ratio.

4. Step 2: oil volume

Required oil volume (cc) = unclamping-cylinder piston area (cm²) × stroke (cm) × 1.2 to 1.3

The 1.2–1.3 factor covers the slight expansion of the high-pressure hose under pressure, which absorbs some oil, so always round up. Then choose the output volume at or above the result (standard 50 to 220 cc, depending on bore). Keep the high-pressure hose under 3 m.

5. Worked example

Required oil pressure 20 kgf/cm², working air pressure 6 kgf/cm²: 20 ÷ 6 ≈ 3.3, so choose the smallest ratio, 6.25:1, and regulate the air down to about 3.2 kgf/cm² (20 ÷ 6.25).
Unclamping-cylinder piston area 117 cm², stroke 0.4 cm: 117 × 0.4 = 46.8 cc, × 1.3 ≈ 60.8 cc, so choose 70 cc.

6. Ø100 theoretical output pressure (kgf/cm²)

Air pressure 4 kgf/cm²: 6.25:1 → 25; 8:1 → 32; 11:1 → 44; 12.7:1 → 51; 16:1 → 64
Air pressure 5 kgf/cm²: 6.25:1 → 31; 8:1 → 40; 11:1 → 55; 12.7:1 → 63; 16:1 → 80
Air pressure 6 kgf/cm²: 6.25:1 → 37; 8:1 → 48; 11:1 → 66; 12.7:1 → 76; 16:1 → 96

Actual output pressure is slightly lower than the theoretical value because of line losses and operating speed.

7. BPF or BPFQ?

The two share the same specifications; BPFQ adds a quick-exhaust valve for faster unclamping, suited to high-speed tool change on direct-drive and built-in spindles, and needs at least 0.6 MPa.

For sizing help, send us the information above through Contact Us and HINAKA will confirm the right specification.

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