Comparing Air Hydraulic Unclamping Cylinders and Mechanical Clamping Systems

Air hydraulic unclamping cylinders and mechanical clamping are often described as alternatives, but on a machining center spindle they work together. The spindle's mechanical system, a stack of disc springs pulling on the drawbar, holds the tool holder clamped during cutting. The unclamping cylinder only acts during tool change, pushing the drawbar against those springs to release the tool. This article explains how the two share the work, and how to choose the right way to unclamp.

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The Mechanical Side: Disc Springs Clamp the Tool

Inside the spindle, disc springs pull the drawbar back and hold the tool holder firmly in the taper. Because the clamping force comes from springs, the tool stays clamped even if air or hydraulic pressure is lost, which is why this is the standard, fail-safe arrangement on machining centers.

• Clamping force: set by the disc spring stack and the drawbar design.
• Always on: no power is needed to keep the tool clamped.
• Released by force: to change tools, something must push the drawbar hard enough to compress the springs.

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The Unclamping Side: Pushing Against the Springs

At each tool change, the unclamping cylinder pushes the drawbar forward, compresses the disc springs and releases the tool holder. As soon as the new tool is in place, the cylinder retracts and the springs clamp again. The unclamping force therefore has to exceed the spring force, with some margin.

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Three Ways to Unclamp

• Pure pneumatic unclamping cylinder: runs on compressed air alone, with no oil to top up and no risk of oil reaching the spindle. HINAKA's DJ2A mounts directly on belt- and gear-driven spindles and completes a tool-to-tool change in 1 to 1.2 seconds.
• Air-hydraulic boosting cylinder: compressed air drives a booster that turns shop air into hydraulic pressure, giving much higher unclamping force in a compact unit. HINAKA's BPT series integrates the booster into the cylinder for belt- and gear-driven spindles; for direct-drive and built-in spindles, a BPF booster supplies a HUC hydraulic cylinder at the spindle.
• Hydraulic power unit: a motor-driven pump supplies the unclamping cylinder directly. It works, but adds a pump, motor and oil tank to the machine.

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How to Choose

• Drawbar force: the higher the spring force to overcome, the more likely you need air-hydraulic boosting.
• Spindle type: belt- and gear-driven spindles take an integrated cylinder at the rear (DJ2A, BPT); direct-drive and built-in spindles need a separate cylinder at the spindle (HUC with a BPF booster).
• Tool change time: where tool change is the bottleneck, the Q versions with a quick-exhaust valve unclamp over 40% faster.
• Coolant through the spindle: the HUC is hollow and works with or without CTS; the DJ2A can be used on CTS spindles with an added bell cover.
• Orientation and environment: for horizontal mounting or humid conditions, the Dual-Oil BPT-D and BPF-D series run on oil in both directions.

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Frequently Asked Questions (FAQ)

Does the unclamping cylinder hold the tool during cutting?

No. The spindle's disc springs hold the tool. The unclamping cylinder only acts during tool change, and retracts once the new tool is clamped.

What happens if air pressure is lost?

Because clamping comes from the springs, the tool stays clamped. Low air pressure affects unclamping, not clamping, which is why a stable air supply matters at tool change.

Do air-hydraulic unclamping cylinders need a hydraulic pump?

No. The hydraulic pressure is generated from compressed air by the booster, either inside the cylinder (BPT) or in a separate booster (BPF).

How much unclamping force do I need?

Enough to overcome the spindle's drawbar spring force with a margin; allowing 20 to 30% extra is common. Send us the drawbar force and unclamp stroke and we will size the unit.

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Conclusion

On a machining center, mechanical disc springs and the unclamping cylinder are partners, not competitors: the springs clamp, the cylinder releases. Choosing the unclamping method comes down to drawbar force, spindle type and tool change time. Contact us with your spindle details for a recommendation.

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Related Articles

BPF or BPT? A Quick Guide to Choosing the Right Unclamping Cylinder for Your Spindle

Air Hydraulic Unclamping Cylinders: Enhancing Efficiency and Precision

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