Engineering / Mechanical Engineering
Disc Springs
A disc spring is a shallow cone of spring steel that flattens under load — enormous force in millimetres of height, and a force–deflection character that can be tuned from nearly linear to dead flat by one geometric ratio.
- Reading time · 4 min
- 6 sections
- Character curves computed
- Stacking arithmetic worked
§1What a disc spring is
A washer pressed into a shallow cone: outside diameter D_e, bore d, material thickness t, and free cone height h. Load it axially and the cone flattens, the material working mostly in bending around its own ring.
The single most important number is the ratio h/t — cone height to thickness. It sets not the size of the spring’s force but the shape of its force–deflection curve, which is what §2 computes. Everything about a disc spring is compact: travel is a fraction of a millimetre to a few millimetres per disc, force runs from hundreds of newtons to hundreds of kilonewtons, and the package is a washer.
Contents§2The force–deflection character
Flattening a cone is not like stretching a coil. The classical (Almen–Laszlo) analysis gives a cubic-flavoured law — normalised below so the geometry’s effect stands alone.
The plateau regime is the celebrated one: a stack held at mid-deflection delivers almost constant clamping force while the joint underneath it creeps, settles or wears — the live-load washer behaviour that keeps bolted joints and tool clamps tight. The snap-through regime, deliberately used, makes bistable elements and tactile switches; accidentally reached, it makes a preload vanish.
Standardised disc-spring series publish D_e, d, t, h and load tables; this page deliberately carries the mechanics, not those proprietary tables. Design against the manufacturer’s data for the series in hand — the character curves above tell you which corner of the catalogue to open.
§3Stacking — series and parallel
One disc rarely suffices, and stacking arithmetic is mercifully simple: nested discs add force, opposed discs add travel.
A single disc gives 1200 N at 1.0 mm deflection. Three series groups, each of two discs in parallel: force 2 × 1200 = 2400 N, travel 3 × 1.0 = 3.0 mm, and the stack’s rate is 2/3 of the single disc’s. Same four washers nested the other way (six in parallel): 7200 N at 1.0 mm — the same steel rearranged into a completely different spring.
Parallel nesting adds inter-disc friction (see §4), which fattens the load–unload loop — sometimes a feature (damping), sometimes a tolerance problem. Keep parallel counts modest (2 – 3 is typical practice) and buy travel with series groups.
Contents§4Friction and guidance
A stack is a column of loose cones: it must be guided, and it will rub.
Stacks run over a guide pin or inside a sleeve; the guide surface should be hardened and lightly greased, with working clearance to the bore or OD kept small so discs stay square. Friction at the guide and between nested discs makes the unloading curve sit below the loading curve — a hysteresis loop whose area is damping. Long series stacks bow like the Columns page’s struts and share load unevenly through friction: common practice limits an unguided run to roughly two to three stack diameters and, for long travels, splits the column with washers or steps in the guide. Re-torque after first seating: a new stack beds in.
Contents§5Where disc springs win
Choose the cone over the coil when the specification says: huge force, tiny travel, no room.
The natural homes: bolted-joint live loading (holding preload through thermal cycling and gasket creep), tool and die clamping, overload protection in presses, spindle drawbars, pipework hangers and bearing preload — all places where a helical spring of equal force would be absurdly large. The disc also offers what no coil can: a tunable curve shape, including the constant-force plateau and controlled snap-through of §2. Its trade-offs are the coil’s virtues in reverse — short life at high working stroke fractions, friction scatter, and travel bought only by stacking.
Contents§6Quick reference
The working core of the page on one card rack.
Geometry
cone h, thickness t
h/t sets the curve shape
Character
h/t ≲ 0.5 → near linear
≈1.4 plateau · beyond → snap-through
Stacking
parallel: F×m · series: δ×n
rate × m/n
Practice
guide the stack, grease the pin
parallel ≤ 2–3 discs
Use when
big force, short travel
live-loading bolted joints