Hydraulic clamping system for machining fixtures: swing clamps, work supports and clamping sequence

01

Read swing clamp force against operating pressure and arm length

Roemheld's block-type double-acting swing clamps with overload protection run at up to 500 bar, but the force reaching the workpiece is read from diagrams of effective clamping force against operating pressure, one line per arm length. Its worked example: 200 bar with the standard 75 mm clamping arm gives an effective clamping force of 7 kN. The complete clamping arm assembly is limited to 200 bar, while the clamping strap assembly with carrier is rated to 500 bar. [1]

Enerpac's STLD92 threaded-body double-acting swing clamp lists a clamping force of 9.0 kN (2025 lbs) at a maximum operating pressure of 350 bar, with a minimum of 35 bar and an effective clamping area of 3.16 cm². [4]

Editorial calculation: 3.16 cm² at 350 bar is 3.16 × 10⁻⁴ m² × 35 MPa ≈ 11.1 kN of piston pull, so Enerpac's listed 9.0 kN is about 81% of the piston force. Roemheld's 7 kN example uses 200 bar, 40% of its 500 bar maximum. [4] [1]

Roemheld warns that hydraulic clamping elements generate considerable forces and that, because of the 90° swing, the exact clamping and unclamping position cannot be fixed in advance; fingers can be crushed in the arm's path. Its remedy is a protection device with electrical locking. [1]

Give the supplier a drawing of the workpiece and the clamping points, the machining forces or the cutting operations, the machine table or pallet interface, the hydraulic supply (pump, booster or rotary coupling) and its pressure, the number of clamps and supports, the swing direction and angle, the time allowed for clamping, and the monitoring signals the machine needs.

02

Allow for swing stroke, clamping stroke and the oil each clamp takes

In Roemheld's pull-type swing clamp, part of the total stroke swings the arm and the rest clamps. Across its three sizes the total stroke is 14, 16 and 20 mm, of which 7, 8 and 9 mm is swing and 7, 8 and 11 mm is clamping; oil volume per clamping stroke is 2.5, 7.3 and 23 cm³ and per return stroke 6.8, 20 and 62 cm³. Standard swing angles are 45°, 60° and 90° ± 2°, and an overload device protects the swing mechanism if the arm strikes an object within its 90° rotation. [1]

Roemheld limits the oil flow rate to 3.2, 10 and 27.7 cm³/s for the three sizes with vertical mounting and standard arms; other positions or heavier arms need a lower rate, set by flow control valves in both the clamping and unclamping lines. Enerpac's STLD92 has 23 mm total stroke with 12 mm clamping stroke and takes 6.88 cm³ of oil to clamp. [1] [4]

Editorial calculation: at its 27.7 cm³/s limit the largest Roemheld clamp needs at least 23 ÷ 27.7 ≈ 0.83 s to clamp. A fixture with eight of the middle size takes 8 × 7.3 = 58.4 cm³ per clamping stroke and 8 × 20 = 160 cm³ to unclamp, which the pump or booster must deliver within the cycle. The clamp count is an assumed example. [1]

03

Size work supports to at least twice the clamping force and sequence them first

Roemheld describes hydraulic work supports as a self-adjusting rest that compensates surface irregularities, deflection and vibration under machining loads; they are not suitable for side loads. Its threaded-body support has a 16 mm plunger, 8 or 15 mm stroke, admissible support force at 500 bar of 6.5 or 9.5 kN depending on version, and a recommended minimum oil pressure of 100 bar; the plunger can be advanced by spring, by air (20 N contact force at 1 bar) or by hydraulic pressure and spring. [2]

For its fail-safe work supports, which hold the plunger position even after a complete pressure loss, Roemheld lists load forces of 8, 20 and 40 kN at 500, 500 and 400 bar. It states that the work support load force should be at least twice the clamping force of the clamp acting on it, met at 200 bar for swing clamp combinations, and that the admissible loads are static, so a safety factor is needed for vibrating machining forces. [3]

Its worked example for size 1933 at 200 bar: load force 6.6 kN minus clamping force 2.8 kN leaves a possible machining force of 3.8 kN including reserve. The supporting-then-clamping sequence is controlled by pressure through a sequence valve set to open above the point where the support and clamp force lines cross. [3]

Editorial calculation: 6.6 ÷ 2.8 ≈ 2.4, so the example meets the twice-clamping-force rule at 200 bar. [3]

04

Swing clamps, work supports and straps in one fixture

The reviewed datasheets describe these elements and what each limits.

05

Send the workpiece, force and hydraulic data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Workpiece: [drawing; material; clamping and support points]
Machining: [operations; estimated cutting forces]
Clamps: [number; type; swing angle and direction; arm length]
Supports: [number; advance type; fail-safe needed]
Hydraulics: [pressure; pump or booster; flow; rotary coupling]
Sequence and monitoring: [sequence valve; position sensing]
Machine: [table or pallet; interface]

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