Direct fastening explained: benefits, design and applications

1. What is direct fastening?

With direct fastening, the internal thread is created directly in the component when the screw is getting fastened – without a necessary thread in the component.
This means:

• No previous thread creation
• No additional fasteners like nuts or clips
• Direct fastening into materials like (light-) metals, plastics or even foam

Result: A reliable, time-saving, and cost-efficient connection
 

2. What are the advantages of direct fastening?

Direct fastening offers multiple key advantages:

• Time and cost savings
• Elimination of hole drilling, thread cutting, and cleaning
• Reduction of assembly times and tool changes
• High process safety
• 100% thread engagement instead of clearance-fit threads like common metric threads
• High self-locking effect → no additional thread-locking mechanism required
• Thread forming causes material hardening in the thread, which increases load-bearing capacity
• No nuts or additional fasteners required

 

3. Which screws are used for which materials?

Different screw types are used depending on the material:

• Plastics: EVO PT®
• Light metals (e.g. aluminium): ALtracs® Xt
• Steels: Spiralform®
• Aluminium profiles: FDS® (flow-drill screw)
• Sheet metals without pilot hole: FDS® (flow-drill screw)
• Thin sheet metals with pilot hole: SHEETtracs®

The right choice directly affects assembly quality and thread load-bearing capacity.
 

4. What should be considered regarding the installation torque?

The installation torque must not exceed the screw's minimum breaking torque.
The minimum breaking torque is the maximum torsional load a screw can withstand without breaking; this value can be found in DIN standards or EJOT manufacturer specifications.
Why?

• To high torque → Risk of screw breakage during thread forming

Recommendation:

• Observe limit values from standards (e.g. DIN) or manufacturer specifications
• Use an additional lubricating coating

 

5. How is the optimal tightening torque determined?

For a robust recommendation, the following applies:
1.    Conduct at least 10 assembly tests until failure
2.    Determine: 

• Installation torque (torque level, where the screw gained head contact)
• Failure torque (maximum torque, where something in the connection breaks)

Optimal position:

• Tightening torque lies midway between the two values

Important rule:

• Safety margin ≥ factor of 2 between installation torque and failure torque

6. Can the tightening torque exceed the minimum breaking torque?

Yes; once the screw head gained contact to the component, additional factors come into play:

• Under-head friction (relieves the screw shaft)
• Clamp load (loads the screw shaft)

Consequently, the minimum breaking torque loses its significance once the screw head gained head contact.
 

7. Which factors are critical for the design?

The following parameters should be considered for a robust direct screw connection:

• Material of the installation part material
• Pre hole diameter and tolerances
• Provision of a pre hole counterbore
• Effective thread engagement depth
• Clearance hole diameter, material, and thickness of the upper component, the clamping part

Tip: Modern design tools significantly simplify the dimensioning process.
 

8. What role does friction play in direct fastening?

Friction directly affects torque and preload force:
Key influencing factors:

• Screw surface finish (e.g. ZnNi, zinc flake, blue zinc plating)
• Screw coatings (lubricants, sealers with integrated lubricant etc.)
• Component coatings (anodised layer, paint etc.)
• Under-head serrations

Practical insights:

• Lubricating coatings reduce torque scatter
• ZnNi without lubricant can lead to galling in aluminium
• Zinc flake coatings are not suitable for cleanrooms

9. What Thread-Forming Speeds Are Recommended?

Typical guidelines:

• Standard: 400–500 rpm
• Small diameters: higher speed possible
• Large diameters: lower speed
• High-temperature-resistant plastics: higher speed

Goal: uniform thread forming without material Damage
 

10. Can stainless steel sheet be fastened directly?

Only to a limited extent:

• A2 / A4 screws: process reliability in stainless steel is not guaranteed
• Alternative: screws made of chromium steel (1.4016), although with reduced corrosion resistance

11. What is a gradient-based tightening process?

The screwdriver system detects the point of head contact based on the rate of torque increase relative to the angle of rotation. After reaching a set numerical gradient limit value, a constant torque or angle of rotation is applied.
Advantages:

• Constant preload force
• Independent of friction variation
• Documentation of tightening parameters possible

Typical for EJOT SpringHead® applications, where the assembly into the plastical deformed area is the goal.

12. What does "fastening beyond the elastic limit” in correlation with EJOT SpringHead® mean?

In this process, the screw head is getting loaded beyond its yield strength on purpose. Objective: maximum clamp load with as few loss as possible
Important requierement:

• Failure reason must be screw breaking; failure of the mother thread is not permitted.

13. Is re-installation possible?

Yes – usually multiple times, up to 10 times or even more.
Tips for a longer service life:

• Start the screw by hand initially to avoid forming a new thread alongside the existing one 
• Apply low axial force at the start

14. Is electrical contacting possible?

Yes, even with coated surfaces:

• Underside serrations (e.g. CONLOK®) penetrate isolating layers
• Thread-forming action secures contact with the clamping part component

Enables reliable electricity flow despite coatings

15. How can the loss of clamp load be minimised under thermal cycling loads?

Using the EJOT SprigHead® heads:

• The EJOT SpringHead® design compensate settling effects
• They minimise the loss of preload force during temperature fluctuations

16. How are bolted joints tested in serial production?

Typical test methods:

• Further-tightening torque (e.g. using a assembly system set to the joints tightening torque and checking whether the screw head moves)

Important:

• Does not provide a direct indication of the level of clamp load 
• Used for process monitoring in manufacturing

17. Which screw for which application?

Light metals with pilot hole:
→ Classic thread-forming screw ALtracs® Xt
Steel plates with pilot hole:
→ Thread-forming screws EJOT Spiralform®
Sheet metals without pilot hole:

• Aluminium up to 5 mm: FDS®
• Steel ≤ 400 MPa: up to 2 mm
• Steel ≤ 800 MPa: up to 1 mm

Sheet metals with pilot hole:
→ SHEETtracs®
Plastics:
→ EVO PT®

18. Conclusion: Why use direct fastening?

Direct screw fastening is a modern, cost-effective, and reliable technology that offers tremendous advantages, particularly in series production:
✔ Reduced assembly costs
✔ High strength and process safety
✔ Fewer components and process steps
✔ Optimal integration possibilities into automatic processes