1. Introduction to Flange Bolt DIN6921
A flange bolt integrates the bolt head and a flange bearing surface as one piece. The flange face is enlarged, and some types come with anti‑serration under the flange. DIN6921 is the common standard for hexagon flange bolts. It is designed to spread clamping load and fit sealing joints. Ordinary hex bolts need independent washers to increase bearing area, which is fundamentally different in structure.
2. Risks of Replacing Flange Face with Ordinary Plain Washer
The bearing area of a standard plain washer is smaller than the integral flange of DIN6921. Under the same preload, surface pressure rises sharply. Soft materials such as aluminum, cast iron and composite gaskets are easy to be crushed.
2.2 Poor centering and offset loading
Separate washers may shift during tightening. The clamping force cannot act vertically on the sealing surface, forming eccentric load. It will cause gasket extrusion and leakage. The integrated flange face of flange bolts keeps the load centered steadily.
2.3 Loosening risk under vibration
Some DIN6921 flange bolts have serrated anti‑slip flange surface. The serrations bite into the mating surface to resist rotation. Ordinary smooth flat washers have no anti‑slip function. Under continuous vibration from pumps, motors and piping, the assembly is prone to loose.
2.4 Assembly tolerance stacking
Using ordinary bolt plus washer adds extra matching clearance. When installing on irregular or countersunk surfaces, the washer may tilt. The integral flange eliminates this tolerance stack‑up problem.
3. Suitable Application Scenarios for DIN6921 Flange Bolts
Joints requiring sealing: hydraulic covers, pipeline flanges, pump housings, gearbox end caps
Thin sheet, cast aluminum and low‑strength workpiece that are easy to crush
Vibration working conditions: compressors, construction machinery, vehicle accessories
On‑site maintenance where space is limited and washer installation is inconvenient
4. Selection & Replacement Rules
If the original drawing or old part is DIN6921 flange bolt, do not replace it with standard hex bolt + plain washer.
When the mating surface is soft, confirm the flange outer diameter of the bolt to guarantee enough bearing area.
For vibration conditions, prefer serrated flange type to improve anti‑loosening performance.
If disassembly and re‑assembly are frequent, check whether the flange bearing surface has indentation or deformation before reuse.
5. Common Misunderstandings
Misunderstanding 1: The washer can provide the same bearing area as the flange face. Correction: Most standard plain washers have smaller outer diameter. Even if the size is close, separated washers cannot match the centering and anti‑slip performance of integrated flange.
Misunderstanding 2: As long as the bolt strength grade is the same, interchange is allowed. Correction: Strength grade controls tensile capacity, while flange face controls surface pressure and sealing effect. Same strength cannot offset structural difference.
Misunderstanding 3: It is acceptable for non‑critical joints to use ordinary bolts and washers instead. Correction: Even non‑pressure joints under vibration will gradually loosen, causing abnormal noise or component dislocation.
6. FAQ
Q1: Can I use ordinary bolt and big size flat washer instead of DIN6921? A1: It is not recommended. Even a large washer is an independent part. It is easy to shift during tightening and cannot replicate the stable bearing of integrated flange.
Q2: What is the function of serrations under the flange of DIN6921? A2: The serrations increase friction with the mating surface, resist bolt rotation and improve anti‑loosening ability under vibration.
Q3: Will flange bolts need extra washers normally? A3: Normally, DIN6921 flange bolts are used without additional plain washers. Add washers only when specified by drawing.
