Bolt pretensioned load calculation 1

[elogesh]

Hai,

Briefly about objective of my e-mail:
a)The stress developed in the plastic part in pretension load
b) Force transmitted in bolt and joint members from the pretension load
____________________________________________________________

One of my roles in the orgnisation is design.For one of our assembly component we are converting aluminium component into plastic(Nylon 6 30% GF). The assembly has three components
1) Body made of aluminum with thickness 12 mm ,thread is provided in this body for bolted connection
2) body made of plastic, earlier it was aluminum with thickness 17 mm
3) Steel plate made of steel of thickness 2 mm

The bolt used for connection is M6.The tigthening torque is 10 N-m. using this and K = 0.3 the pretension force is approximately 5600N

The plastic is provided with an metallic(steel) insert with inner diameter 6.5 mm and outer dia 9 mm and height 16.5 mm.This metallic insert is placed when this plastic component molded.

I had some knowledge in bolted connection.

As per usual practice I considered bolt and joint members are parallel and hence the deflection is same for bolt and joint members and the total force is resolved between the bolt and joint members depending upon their stiffness.

Hence, I calculated bolt stiffness and joint member stiffness. From this, I calculated the force on bolt and in joint members.

Force on bolt = 100 N (approx)
Force on joint members=5500 N(approx)

Then I considered one more case that is plastic part without metallic insert.Surprisingly
The force on bolt = 600 N(approx)
The force on Joint members = 5000 N(approx).
I expected joint members willl be subjected to higher loads compared to earler case of plastic part with metallic insert.
Later, I thought that it may be due to that in parallel springs, rigid spring take more load and flexible spring takes less load to keep deflection same.
Hence in plastic without insert the stiffness of joint member comes down and hence bolt takes more load compared to earlier case of plastic part with insert.
Whther the reasoning is right? This is my first query.

Second query is for calculating the stress developed in plastic part due to pretension load.How to calculate that. the difficulty is what value of cross-sectional area, I have to account.

Third query, Is there any web-site related threaded fasteners with plastic parts.

Thanks for patience in reading the thread.

Have a great weekend........

Regards,
Logesh.E

 

[CoryPad]

It appears that you are using the relative stiffness ratio to determine bolt and joint forces. This applies to external forces, not the pretension. The bolt and joint both see a 5 600 N force during tightening.

The stress in the plastic part can be estimated (conservatively) as being equal to the force divided by the contact area of the screw head. If you have a metallic insert, then the plastic has essentially zero stress due to pretension, but will have stress due to shear and bending forces when external forces are applied.

I don't know of a site that specializes in plastic parts. Read FAQ725-600 and FAQ725-215 for good info sources.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[elogesh]

Hai,

Thanks cory and a star for your helpful post.

Regards,
Logesh.E