crevice-free bolted joint
Crevice-free bolted joints in hygienic equipment
By ISOKLAMP Engineering, Inc. Editorial Team · Updated
Why crevice geometry is the constraint in hygienic bolting
In food, dairy, beverage and pharmaceutical equipment, the bolted joint is not judged on clamp force alone. It is judged on whether it can be cleaned in place. 3-A Sanitary Standards and EHEDG guidance both treat crevices, undercuts and metal-to-metal gaps as harbourage sites, because clean-in-place fluid velocity drops to effectively zero inside them and biofilm establishes.
That single constraint disqualifies most of the securing devices an engineer would otherwise reach for.
| Securing method | Crevice-free | Why |
|---|---|---|
| Split (spring) washer | No | Open gap at the split, permanently |
| Serrated / external tooth washer | No | Tooth roots are unreachable undercuts |
| Wire locking | No | Wire-to-hole interface, and the wire itself |
| Nyloc nut | No | Insert-to-thread interface traps fluid |
| Castellated nut and pin | No | Slot geometry and the pin hole |
| Double nut | Marginal | Nut-to-nut interface is a thin annular gap |
| Wedge-lock washer pair | Marginal | Cam faces interlock, radial gap at the pair line |
| Plain washer, torque only | Yes | But no securing function at all |
The result is that a great deal of hygienic equipment is bolted with plain washers and a torque spec, then retorqued on a maintenance schedule, because everything that would secure the joint properly cannot be cleaned.
Reference joint used throughout
Every figure in this article refers to the same reference joint, so numbers are comparable across articles and against your own calculations.
| Parameter | Value |
|---|---|
| Bolt | M16 × 2,0, property class 10.9, to ISO 898-1 |
| Assembly preload F_V | 70,0 kN |
| Clamp length | 48 mm, steel on steel |
| Bolt stiffness k_S | 1,04 × 10⁹ N/m |
| Member stiffness k_P | 5,71 × 10⁹ N/m |
| Load factor Φ | 0,154 |
| Transverse test | DIN 25201-4:2010-03 Annex B, 2 000 cycles, ±0,45 mm slip |
Stiffnesses are calculated to VDI 2230 Sheet 1 using the standard cone-of-compression method.
The second problem: hygienic joints are Mode B joints
Hygienic joints are exactly the joints most exposed to non-rotational preload loss.
- Gaskets. PTFE, EPDM and silicone all creep under sustained compression. A PTFE gasket can lose 20 to 60 µm of thickness over its first months in service.
- Thermal cycling. CIP at 80 °C and SIP at 121 to 134 °C, cycled daily against ambient. Stainless expands at 16 to 17 × 10⁻⁶ /K, and the stack ratchets. See CIP and SIP thermal cycling.
- Soft members. Stainless is softer than the class 10.9 or A4-80 fastener, so embedment at the faying surfaces is higher than in a steel-on-steel joint.
A joint losing clamp force through gasket creep and thermal ratcheting cannot be helped by a device that only stops the nut turning. It needs length back. That is the entire argument for the self-tightening washer in this sector.
What crevice-free actually requires
For a two-part washer to be genuinely crevice-free rather than merely smooth-looking:
- Continuous circumferential contact at both the bolt-head interface and the member interface. No radial slot, no split, no interrupted bearing.
- No external undercuts. Every external surface must be line-of-sight accessible to a spray ball at cleaning fluid velocity.
- Radii, not corners. 3-A guidance calls for internal corner radii that a cleaning film can maintain contact around. The ISK-16 carries a 1,45 mm chamfer at the top edge.
- Passivated surfaces throughout. Any securing device that scratches, gouges or embeds into the stainless passive layer creates a corrosion initiation site, which becomes a pitting site, which becomes a crevice. This is why serrated washers are unusable here regardless of geometry. See no surface damage.
- Self-draining orientation. No horizontal upward-facing surfaces that pool.
The ISK-16 places its working mechanism entirely inside the two-part body. The cam faces, the helical ramp and the constant-torque element are internal. The external envelope is a smooth cylinder with a chamfered top edge and a single indicator window.
Stainless galling and why it matters here
A4-80 and A2-70 stainless fasteners gall. Once the nut and thread cold-weld, the joint cannot be disassembled without destroying the fastener, and the torque-to-preload relationship becomes unpredictable, which means the preload you specified is not the preload you got.
A washer that carries the securing function on internal cam faces rather than on the thread takes the securing duty off the galling-prone interface entirely. Detail on stainless galling.
Performance in a hygienic stack
Analysis of a representative hygienic joint, M16 A4-80, 3 mm PTFE gasket in the stack, 60 CIP/SIP cycles between 20 °C and 134 °C:
| Method | Residual clamp force after 60 cycles |
|---|---|
| ISOKLAMP CFR | 97,6 % |
| Wedge-lock pair | 78,2 % |
| Plain washer, torque only | 51,9 % |
| Conical spring washer | 63,4 % |
ISOKLAMP figures are predicted by finite-element analysis coupled to VDI 2230 Sheet 1 joint mechanics with a measured PTFE creep model. Baselines are calibrated against published gasket-relaxation data, sourced on the test data page.
The gap widens with cycle count, because gasket creep is cumulative and only one of these methods has a reserve to spend against it.
Specifying it
Full application detail, gasket compatibility and surface finish data on hygienic bolting. Material specifications and dimensional drawings on the specifications.
Frequently asked questions
What makes a bolted joint crevice-free?
A crevice-free bolted joint presents no gap, undercut or entrapment site on any external surface where product residue or cleaning fluid can lodge. In practice this requires continuous circumferential contact at both bearing interfaces, no external undercuts, generous radii rather than sharp internal corners, fully passivated surfaces, and a self-draining orientation.
Which locking washers are acceptable for 3-A sanitary equipment?
Split washers, serrated and external-tooth washers, wire locking, castellated nuts with pins, and nylon-insert nuts all create harbourage geometry and are unsuitable. Wedge-lock washer pairs are marginal because the pair line presents a radial gap. A securing device whose working mechanism is entirely internal to a smooth external envelope avoids the problem.
Why do hygienic bolted joints lose clamp force so quickly?
Hygienic joints combine three non-rotational loss mechanisms. Gaskets in PTFE, EPDM or silicone creep under sustained compression, losing 20 to 60 micrometres. CIP and SIP thermal cycling between ambient and 134 degrees ratchets the stack. Stainless members are softer than the fastener, so embedment is higher than in steel-on-steel joints. None of these involves the nut rotating.
Do serrated washers damage stainless steel?
Yes. Serrated and external-tooth washers work by biting into the bearing surface, which breaks the passive layer on stainless steel. The resulting exposed sites initiate pitting corrosion, and a corrosion pit is itself a crevice. This rules them out of hygienic service on both the geometric and the metallurgical argument.
How does stainless galling affect preload in hygienic joints?
A4-80 and A2-70 stainless fasteners are prone to thread galling. Once cold-welding occurs the torque-to-preload relationship becomes unpredictable, so the assembly preload achieved differs from the preload specified, and the joint cannot be disassembled without destroying the fastener. Carrying the securing function on internal cam faces rather than on the thread removes the securing duty from the galling-prone interface.
Take it further
- The ISOKLAMP technical report covers the full derivation, the geometry, and every dataset behind these figures.
- Full residual clamp force dataset gives the residual clamp force numbers for ten securing methods, with sources.
- Contact sales puts you in touch with the engineering team. Bring us a joint that keeps failing and we will run a VDI 2230 Sheet 1 analysis on it.
Engineering questions go to engineering@isoklamp.com. An engineer answers, not a form.