Damping cylinder for reclining seat and backrest linkages: an oil-hydraulic strut that brakes the backrest, rated by damping force at a test speed rather than by spring force

01

Damping force in newtons at 100 mm/s, in tension or in compression

A damping cylinder is ordered by the force it produces at a stated speed. SUSPA's HD25 standard programme has a 25 mm tube and an 8 mm piston rod, hydraulic strokes of 91 to 206 mm and damping forces from 100 to 4,000 N, each measured at a linear test speed of 100 mm/s; every catalogue variant damps in one direction with a stated force and gives less than 100 N in the other. [1]

HAHN Gasfedern gives a wider span for its closed oil dampers, 5 to 2,500 N, damped in tension, in compression or both, and offers an HB series whose damping force is set on site by turning the piston rod, for applications where the force cannot be fixed in advance. [3] The same maker warns that the damping behaviour of an oil brake changes considerably with temperature, and its sizing form asks for the operating temperature, the load's weight and centre of mass, the angles of travel and the time the motion should take. [4]

Editorial calculation: a force stated at 100 mm/s says how hard the cylinder pushes back at that speed only. A 650 N cylinder [1] at its test speed dissipates 650 N × 0.1 m/s = 65 W; driven over its full 105 mm hydraulic stroke at that speed it absorbs about 650 × 0.105 ≈ 68 J. If the linkage moves the rod faster or slower than 100 mm/s, the force is different, so ask for the force–speed curve when the backrest speed matters.

Give the damper maker the direction to be damped (backrest going back, or coming up), the force or the time the travel should take, the rod speed the linkage produces, and the temperature range of the room or vehicle.

02

Idle stroke before damping, one-way or both-way types, and the vertical mounting rule

In three of SUSPA's HD25 rows the mechanical stroke is longer than the hydraulic stroke: 129 mm against 105 mm, 188 mm against 153 mm and 206 mm against 166 mm. [1] Editorial calculation: those cylinders move 24 mm, 35 mm and 40 mm before the oil starts to brake. HAHN notes on its oil brake sizing form that, because of their function, oil brakes have an undamped free stroke. [4]

Stabilus divides its STAB-O-SHOC motion dampers by this property. The HD 15 is a position-dependent standard damper for low damping forces with an idle stroke; the GD 15 adds an extension force; the GD 15 SP transmits force in both directions with no idle stroke, in any mounting position, also with an extension force. The HD 24/29 family repeats the pattern for high damping forces, with a separating piston in the GD versions. [2]

Stabilus explains the mounting rule behind the names: its simple motion dampers are built for a position-dependent, vertical installation, and only with an additional closed separating element can they be fitted in any position. It also says bypass grooves can vary the damping over the stroke, path-dependently, while the required force is defined in one direction of movement. [2] Editorial note: in a recliner this decides where the cylinder can sit, since a strut that lies close to horizontal under the seat needs a separated, any-position type.

State the angle the cylinder lies at in the fully upright and fully reclined positions; Stabilus rates its simple, position-dependent motion dampers for vertical mounting only.

03

Driving the cylinder from a rocker arm when the backrest only tilts a few degrees

Geven's European patent application EP 4 292 875 A1, for reclining passenger seats, describes the problem directly: a hydraulic cylinder that damps the backrest needs a stroke of a certain length to work well, which is hard to obtain when the backrest reclines by only a small angle. [5]

Its answer is a lever hinged on a pin fixed to the seat frame and coupled to the lower end of the backrest frame, forming a rocker arm that swings through a fixed angle between upright and reclined; the cylinder is pinned between that rocker arm and the fixed frame, so the rocker arm's travel can be wider than the short travel at the backrest's lower end. One cylinder end has a threaded section with an adjusting nut so its position at the hinge can be set and the cylinder removed easily, and the other end clamps to a frame cross bar at an adjustable position, which lets the fitter make the motion more damped or replace only a worn cylinder. [5]

The same document allows two mirror-image cylinders, one on each side, because an asymmetric push on the backrest would twist the two frame members against each other, and a cylinder with an internal spring that returns it to the upright position when the backrest is released. [5] HAHN gives a matching rule for flaps: it recommends two dampers, one on each side, and says a single off-centre damper can let the part warp or fail to close fully at the fixing point. [4]

04

Data for a recliner damping cylinder

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

Motion: [backrest recline; backrest return; footrest; seat slide]
Damped direction: [tension; compression; both]
Force: [N at stated rod speed, or time for full travel]
Stroke: [hydraulic stroke; acceptable idle stroke]
Lengths: [extended and compressed length; pin centres]
Mounting: [angle in upright and reclined positions; position-independent type needed?]
End fittings: [clevis; fork head; ball joint; threaded end with nut]
Environment: [temperature range; cycle count expected]
Number per seat: [one central; two, one each side]

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