Are Plumber Blocks Strong Enough for 880 Nm of Torque?

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The discussion centers on the strength and stability of plumber blocks used in industrial fans, particularly regarding their ability to withstand 880 Nm of torque. The participants debate the necessity of pinning the blocks to prevent movement, with one member arguing that the blocks have remained stable for 20 years without issues. Calculations suggest that the tension generated by the bolts is significantly higher than the force acting on the blocks, leading to questions about the relationship between torque, friction, and the effectiveness of the bolts. Concerns are raised about potential loosening over time due to thermal expansion and the implications of using lubricants on bolt tension. Ultimately, the conversation highlights the complexities of mechanical fastening and the importance of considering both theoretical calculations and practical experience.
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hello forum members,

I,m new to this forum and was hoping some one can help me?

I work with industrial fans, we use a steel built frame with the shaft & bearings located in an housing called "plumber blocks" that are on top of the steel frame. The blocks are bolted down using 16mm bolts that we use 220 Nm on a torque wrench to tighten down, there are two bolts in each block (4 in total, block either end of shaft) so that's 880 Nm in total. The electric motor is bolted on the steel frame on the side with pulleys and rubber belts, the tension of the belts for say a 30kw motor will be approx 60,000 N. The cosine angle = approx 30 degree's so the tension on the blocks should be approx 51961 N.

I have a friendly argument with my boss saying that the blocks must be pinned (this means inserting a 6mm pin either side of the block tightly to stop movement), he says no, they have never moved once in the 20 years he has built them.

My question isn't who is right or wrong, but is there some way of knowing what 880 Nm is compared to the Cosine value of 51961 Newtons?
 
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Turv said:
16mm bolts that we use 220 Nm on a torque wrench to tighten down.
Without the thread pitch, and the friction factors, there's no way to know how much tension the bolts generate with 220nm of torque applied to tighten them, but it's bound to be a large number (well over 100,000 Newtons). 220Nm / 8 mm radius = 27500 N of force at the surface of the bolt in the direction of rotation. If friction is zero, then the threads are similar to a frictionless thin wedge with a side force of 27500N's of force. If friction is zero, and if I've thought this out right, then the tension = 27500N / (sin(θ) cos(θ)) where θ is the angle of the threads. If the pitch was 10 to 1 (about 5.7 degrees), then tension ~= 277000N.
 
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Thanks a lot jeff,

Question? was that last calculation based on 1 bolt (220 Nm) or the total 880 Nm ?
 
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Turv said:
Thanks a lot jeff,

Question? was that last calculation based on 1 bolt (220 Nm) or the total 880 Nm ?

one bolt.
 
Jeff,

I'm not trying to be awkward or show disrespect to your calculations, these blocks are slotted and can move, i find 51961 N which is 5302 Kg of tension hard to comprehend against 4 bolts of 220 Nm each? am i thinking wrong?

Also you mention friction which i understand, would i be right in saying if we oiled the bolts then we would have more Newtons vs Newton meters?

Also another question? bolts loosen with coefficients of linear/cubical expansion, of which bolts go loose! fact! so the Nm will obviously come down over time.

So i am right that the blocks need to be pinned, although this is not my original question.
 
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Jeff is in the right order of magnitude. I got ~860 kN of tension per screw using the http://www.buyblueprint.com/article/48/" . However, as you can see in the equations the force depends very critically on the diameter and pitch of the screw.
 
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Turv said:
I'm not trying to be awkward or show disrespect to your calculations, these blocks are slotted and can move, i find 51961 N which is 5302 Kg of tension hard to comprehend against 4 bolts of 220 Nm each? am i thinking wrong?
The torque on the bolt isn't directly relateable to the force holding the parts together - as was shown, it is a function of the torque, firction, and thread pitch.

...And you can't just multiply it by 4 - that's a meaningless number.
Also you mention friction which i understand, would i be right in saying if we oiled the bolts then we would have more Newtons vs Newton meters?
If you oiled the bolts, you'd be able to turn them further for the same torque, generating a higher tension in the bolt.
So i am right that the blocks need to be pinned, although this is not my original question.
I'm an HVAC engineer, but I don't understand the configuration you are describing or what you mean by pinning the blocks down - bolts are pins, aren't they? Do you have a picture or diagram you could show us?
 
Russ,

pins are split dowl pins which close up tight when you bang them in, you have to drill the blocks and the steel frame then bang the pins in with an hammer, it stops any movement sideways.
 
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Turv said:
he says no, they have never moved once in the 20 years he has built them.

That's a solid argument to consider, no ? :smile:
If you think that the force holding the bolts is way insufficient to keep the blocks in place, then how come that it didn't move in all these years ?
 

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