Tighten a bolt against a face that is not square to its axis and something has to give. The bolt head cannot sit flat, so it contacts on one edge, and the bolt is bent as it is tensioned. That bending stress adds to the tensile stress from preload, concentrates under the head where the fillet already raises stress, and can take a bolt to failure at a fraction of the load it is rated for.
A spherical washer pair removes the bending without removing the preload.
The Problem They Solve
Angular misalignment between a bolt and its seat turns up constantly:
- Tapered flanges on structural steel sections.
- Cast and fabricated parts where the bolt face was never machined square.
- Sloped or inclined mounting surfaces on brackets and beams.
- Machine bases being levelled, where the foot ends up at a slight angle.
- Clamping on a slot where the clamp tilts as it takes load.
A plain washer does nothing here - it simply tilts with the head and transmits the same edge loading. The bolt still bends.
Bending is the hidden failure mode. A bolt tightened against a 2° slope is carrying a bending moment on top of its preload every moment it is in service, and under fatigue loading it will crack at the head fillet or the first engaged thread long before its rated life.
How the Pair Works
The system is two parts with matching spherical radii:
- The conical seat sits against the sloped surface. Its upper face is dished - a section of a sphere.
- The spherical washer sits on top of it, its underside domed to the same radius.
- The bolt passes through both. As it is tightened, the two halves slide on each other until the spherical washer's top face is square to the bolt axis.
The bolt head now bears on a flat face perpendicular to itself, and the angular difference is absorbed between the two washer halves instead. Load is spread evenly around the head, and the bolt carries pure tension.
The usual correction range is around 3°. That sounds small, and it is - these are not universal joints. Beyond about that, the halves run out of bearing area and the contact stress between them rises sharply; the answer then is to machine the seat, not to stack more washers.
Forms C, D and G
| Form | What it is | Use it for |
|---|---|---|
| C | Spherical washer - the convex (male) half, with a domed underside | Sits under the bolt head or nut; always the upper part of the pair |
| D | Conical seat - the concave (female) half, with a matching dished top | Plain, closed round holes only |
| G | Conical seat - concave, with a larger, flatter bearing face | Slots, oversize holes and anywhere Form D would not be fully supported |
Forms C and D (or C and G) are always used as a matched pair - the radii must match, so do not mix brands or sizes within one joint.
The naming trips people up, so it is worth being blunt about it: Form C is the convex, ball-shaped one that goes under the bolt head. Forms D and G are the concave seats that go against the workpiece. If a drawing calls for "DIN 6319 C and D", it is asking for one of each - a pair - not two of the same thing.
Choosing Between D and G
Both D and G are concave seats; the difference is the bearing face.
- Form D has a smaller outside diameter, sized for a plain, closed, round hole. Its bearing face is fully supported by the material around the hole.
- Form G has a larger, flatter bearing face. Use it over a slot, over an oversize or clearance hole, or anywhere Form D would overhang unsupported material and dig in under load.
This is the single most common specification error with these parts: fitting a Form D seat over a slot. Under preload the seat is bridging a gap, it deforms into the slot, and the alignment you fitted it for is lost - along with a good deal of preload.
Fitting Them Correctly
- Concave against the work, convex under the head. The seat goes down, the spherical washer goes on top. Fitted upside down, the pair cannot articulate and you have simply added two washers.
- Match the pair. The radii must match exactly, so use a matched set of the same size and series. Do not mix brands or sizes within a joint.
- One pair per joint, normally. A pair under the head handles the misalignment at that end. Add a second pair under the nut only when both faces are out of square.
- Keep the mating faces clean and lightly lubricated. The halves must be able to slide as the bolt is tightened. Grit between them locks the joint at the wrong angle.
- Check the clearance hole. The bolt still has to pass through at an angle - a tight clearance hole will contact the bolt shank and reintroduce the very bending you are trying to remove.
- Tighten normally. The washers do not change the torque-tension relationship materially, though the added sliding interface means torque-controlled tightening has slightly more scatter than usual.
When to Use Them
Use a spherical washer pair when: the bolt seat is out of square by up to about 3°, the joint is highly loaded or fatigue-loaded, or the surface angle cannot be machined out economically - structural connections, bridge and crane assemblies, machine base levelling, angled brackets, construction equipment.
Do not use them when: the misalignment is more than a few degrees, you need precise axial positioning (the pair can slide slightly under load), or the joint is lightly loaded and a spot-face would fix it for less money. And note they are a bending fix, not a locking device - if the joint also vibrates loose, that needs a separate solution.
Need Spherical Washers?
DIN 6319 spherical washers and conical seats, M8 to M100, in carbon steel and SS 304 - ex-stock Mumbai.
View Washer RangeFrequently Asked Questions
What is the difference between DIN 6319 Form C and Form D?
Form C is the spherical washer - the convex, domed half that sits under the bolt head. Form D is the conical seat - the concave half that sits against the workpiece. They are used as a matched pair, so a drawing calling for "C and D" wants one of each, not two of the same part.
When should I use Form G instead of Form D?
Form G is a conical seat with a larger, flatter bearing face, for use over slots, oversize holes, or anywhere a Form D seat would overhang unsupported material. Fitting Form D over a slot is a common error - under preload the seat bridges the gap, deforms into the slot and loses both alignment and preload.
How much misalignment can a spherical washer correct?
Around 3°. Beyond that the two halves run out of bearing area on each other and the contact stress rises sharply. If the misalignment is larger, machine or shim the seat square rather than relying on the washers.
Which way round do spherical washers go?
The concave conical seat goes against the workpiece, and the convex spherical washer sits on top of it under the bolt head. Fitted the other way round, the pair cannot articulate and you have simply added two washers to the stack without correcting anything.
Do I need a spherical washer at both ends of the bolt?
Only if both faces are out of square. One pair under the head is normal and handles the misalignment at that end. Add a second pair under the nut when the nut face is also angled.
Can a spherical washer replace a lock washer?
No - they solve different problems. A spherical washer pair removes bending caused by an out-of-square seat; it does nothing about a joint that vibrates loose. If your joint has both problems, address them separately.