Belleville washers vs a one-way take-up

A Belleville stack is the standard answer to preload relaxation and it works: it retains 68,7 % of clamp force at 150 µm of stack shortening where a rigid stack is at zero. The cost is fatigue. A disc spring is compliant in both directions at all times, so it raises the load factor Φ from 0,21 to 0,82, meaning the bolt absorbs roughly four times more of every external load cycle. ISOKLAMP CFR is compliant in one direction only and latches afterwards, holding Φ at 0,27.

What a disc spring actually does

A Belleville washer stores elastic energy in a conical shell. As the stack shortens through embedment, creep or gasket relaxation, the cone flattens slightly and pays out deflection, so the bolt loses far less stretch than it would in a rigid stack. Nothing is recovered — the loss is absorbed by a spring that was compressed at installation.

The load factor, and why it matters

Load factor comparison between a rigid stack, a Belleville stack and ISOKLAMP CFR — Model-predicted. VDI 2230 Sheet 1 load-factor model.

Load factor Φ — the share of an external load cycle carried by the bolt.
Configuration	Load factor Φ
Rigid stack	0,21
Belleville stack	0,82
ISOKLAMP CFR	0,27

A higher Φ means the bolt, not the clamped parts, takes the alternating component of every external load. That is the fatigue penalty, and on a dynamically loaded joint it is the reason disc springs are not used everywhere.

Nonlinear vs linear compliance

Nonlinear stiffness characteristic: soft in take-up, stiff under load reversal — Model-predicted characteristic. Soft in the take-up direction, stiff once latched.

A disc spring has one stiffness in both directions, permanently. ISOKLAMP CFR is soft only in the direction that recovers stack height, and stiff in the direction that carries load — which is why it can compensate without paying the full Φ penalty.

Where a disc spring is still the right answer

On low-fatigue-duty joints such as busbar terminations, the Φ penalty matters much less, and a €0,50 disc spring is hard to beat on cost. Our advantage there narrows to compensation range, latching and the indicator. If the joint is static, cheap to inspect and not fatigue-critical, specify the disc spring.

Side-by-side

Rigid stack, Belleville stack and ISOKLAMP CFR compared. ISOKLAMP figures are model-predicted.
	Rigid stack	Belleville stack	ISOKLAMP CFR
Compensates relaxation	No	Yes	Yes
Latches after compensating	—	No	Yes
Load factor Φ	0,21	0,82	0,27
Residual at 150 µm shortening	0,0 %	68,7 %	97,0 %
Resists rotational loosening	No	No	Yes
Reserve indication	No	No	Yes

Questions

Do Belleville washers stop preload loss?: They compensate it rather than stop it. A disc spring stack retains 68,7 % of clamp force at 150 µm of stack shortening where a rigid stack is at zero. It never latches, so the joint stays permanently compliant.
What is the fatigue cost of a disc spring?: A disc spring is compliant in both directions at all times, so it raises the load factor Φ from 0,21 to 0,82. The bolt then absorbs roughly four times more of every external load cycle. ISOKLAMP CFR is compliant in one direction only and latches afterwards, holding Φ at 0,27 (model-predicted).
When is a Belleville stack still the right choice?: On low-fatigue-duty joints such as busbar terminations the Φ penalty matters much less, and a €0,50 disc spring is hard to beat on cost. Our advantage there narrows to compensation range, latching and the indicator.