Help please to understand 'foot pounds' ( and fitting my PSS seal)

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pcatterall

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My PSS seal has to compress the rubber bellows by the application of '6 foot pounds' how would this figure translate into the force my poor arthritic hands will require when up side down in the bilges.

Previous postings on this forum suggest that made up levers will be required but others report that stiffer bellows are now being used and that the supplied figures may not be correct.
There is a figure quoted for how much the bellows should be compressed ( 3/4" in my case) I can achieve this on the kitchen table applying about 40lbs ( kitchen scales!) and wonder if the foot pounds figure supports my crude experiment.

A eureka moment has occurred!! Compressing the bellows while fitting the face part of the seal has been debated at length. While at the kitchen table I thought " why not compress the bellows to the correct length ( on the galley table) and tie them there with some thin line using the jubilee clips to tie to, then slide the bellows onto the shaft followed by the face part, secure the face part against the carbon face and loosen the 'string'............... I think it would work ok?

A simple explanation of the foot pounds would be welcome as will all congratulations on the fitting idea!!
 
It's what is technically known as a 'moment' or turning force - think of a lever 1 foot long with a force of 6 pounds being applied. Nowadays one would use newton.metres or Nm, and this and so 6lbs.ft would be approx 6/2.2 x 1/3 x 9.8 Nm or about 10Nm.
 
I don't think your compression trick would work. What I do is fit the thing together with everything ready to go and the rotor lightly against the bellows. Fit a jubilee clip close behind the rotor and do it up tight. Mark the position of the rotor with permanent marker to show the starting point. Insert a lever between the rotor and the jubilee clip and lever till the bellow are compressed 3/4in. Nip up the allen key whilst keeping the pressure on the lever - 3 hands do help here. Finish the installation as per the instructions.

If you move the jubilee clip within 1/4 in of the rotor it does dual service as a back up and warning should the rotor move on the shaft.

The 6 foot lbs is the figure for tightening up the grub screws on the shaft not anything to do with compressing the bellows. See paragraph 13 of the instructions. It means that if the allen key was a foot long you should lean on it with 6lbs of force; of course the key is only c 3in long so you should try to put 24 lbs on it. In plain English: give it a good tweak till it hurts your hand.
 
Thanks Doug, Yes my eyes had jumped to the next line when looking at the torque to be applied. I do have a sort of rule of thumb in fact for the 'turning sort of torque' but couldn't see how it translated into 'thrusting force'.... now I understand.... it doesn't!!

You don't explain why you think my 'compress and retain with string' would not work but I do like your method better! I had dismissed this originally thinking that I could somehow spoil the O ring fittings but on reflection they are under no more stress under the load of the bellows on the face than just sitting on the shaft. I guess that a curved 3/4" shim could replace the third hand while the grub screws were tightened.
Thanks again
 
pcatterall said:
A simple explanation of the foot pounds would be welcome as will all congratulations on the fitting idea!!
Click to expand...

Joker said:
It's what is technically known as a 'moment' or turning force - think of a lever 1 foot long with a force of 6 pounds being applied. Nowadays one would use newton.metres or Nm, and this and so 6lbs.ft would be approx 6/2.2 x 1/3 x 9.8 Nm or about 10Nm.
Click to expand...

Strictly speaking a foot-pound is a unit of work or energy . It is the work done when a force of 1 pound acts through a distance of 1 foot. For example when raising a weight of 1 pound by 1foot against gravity.

To avoid confusion the term pound-foot can be used when referring to torque or turning moment. It is the result of applying a force of 1 pound to a lever 1 foot from the pivot point or fulcrum. For example it is the torque applied to a nut or bolt when a force of 1 pound is applied to the end of a spanner or wrench 1 foot long.


HOWEVER the above aside

In connection with fitting a PSS seal the figure of 6 foot-pounds ( Or pounds-feet !) refers to tightening of the two grub screws ( the Yanks call them set screws) against the shaft . IT HAS NOTHING to do with the extent to which the bellows are compressed.

While keeping the bellow in the “compressed” position, tighten the two (2) set screws against the shaft with the provided allen wrench (Use approximately 6 foot pounds of torque for shafts ¾” to 1⅜” and 8 foot pounds of torque for shafts 1½” to 3¾”​

I would recommend reading the instructions for installing the PSS seal more carefully.


( Note for pedants: .... read "pounds" in the first part of my post as "pounds-force". There is no need to tell me about it! )
 
I think the string trick could be made to work if it were secured to the corrugations at each end of the bellows. However getting the pressure even and correct and then not disturbing it whilst fiddling in the depths of the engine room might be a frustrating business.

The compression is not great and I found it quite easy, tho I must admit that I have no experience of the new, harder bellows that have been spoken about. Your 40lb kitchen scale force does sound more than I expected.
 
Did I miss something or is there a reason why nobody seems to have suggested using a torque wrench?

iu
 
VicS said:
Strictly speaking a foot-pound is a unit of work or energy . It is the work done when a force of 1 pound acts through a distance of 1 foot. For example when raising a weight of 1 pound by 1foot against gravity.
Click to expand...

It is also a unit of torque, because angles are dimensionless and therefore work done = torque x angular displacement gives the same dimensions for both work done and torque.
 
6 Ft lbs on a small allen key is about as hard as you could possibly turn it by hand, when upside down in said bilge. Assuming the allen key is 4" long then 18 LBs on the end would = 6 ft LBs

But this is the grub screws, not the bellows as mentioned.

Here's my method for compressing the bellows without busting a few bones and muscles.

Seal assembled and rotor pressed loosely up to the bellows / stator part. Place a jubilee clip loosely on the shaft above the rotor ready to nip down onto the shaft.

Measure the distance from the coupling face to the rotor face with a rule or caliber- distance A

Now the part I prepared earlier - an over length nut used for unistrut assembly either M10 or M8 and a short piece of stud bar cut 1mm less than distance A. Screw the stud bar into the nut until the nut is flush with the thread end. A normal nut can be used but will take several resets and some suitable pad pieces.

Position the stud bar between the coupling (or a conveniently positioned G Cramp) and the rotor then holding the stud with a pair of mole grips unwind the nut with a spanner so it bears against the rotor and as it un-screws it will push the rotor into tension against the bellows. Keep unwinding until the distance A has increased by the compression distance.

If your nut is shorter than the compression distance lock the rotor with a jubilee clip, then place a spacer or second nut the other end of the stud and crying on winding the rotor toward the bellows until the correct distance is achieved.

BTW the jubilee clip provides a secondary security after the grub screws are nipped up.
 
superheat6k said:
6 Ft lbs on a small allen key is about as hard as you could possibly turn it by hand, when upside down in said bilge. Assuming the allen key is 4" long then 18 LBs on the end would = 6 ft LBs
Click to expand...

In the instructional video they dont mention the 6 ftlbs. They just say do them up tight!

In the printed instructions they suggest the use of a vice-grip if the long end of the allen key has to be inserted into the screw
 
VicS said:
In the instructional video they dont mention the 6 ftlbs. They just say do them up tight!

In the printed instructions they suggest the use of a vice-grip if the long end of the allen key has to be inserted into the screw
Click to expand...
I used a short ring spanner as a lever - not mush space for much else ! As it was I could just turn the grub screw one flat at a time all the way in - deep joy when that job was finished.
 
6 ft lbs is sod all. Even with an Allen key it is not mightily tight. The purpose of torque settings is to very slightly stretch the threads, having overcome the friction.
If you do manage to find a torque wrench that measures very low settings you'll be amazed how slight the force needed is to achieve a setting like 12 ft lbs.
 
The principal point about tightening the grub screws is that they should indent the shaft so that the stainless steel collar cannot move back, allowing water into the boat. One of the perennial complaints about PSS seals is that this has happened, whereas the reality is that the thing was never assembled properly in the first place. As has been said, using the supplied Allen key do it up as tight as you can manage. There is nothing that will harm by this, but don't try it using a metric key instead, which will turn inside the grub screw at relatively low torque. Then put the second grub screw in and do the same.

If ever you remove the screws make sure when replacing that the point is still good before replacing, so that the indent can take place.
 
vyv_cox said:
The principal point about tightening the grub screws is that they should indent the shaft so that the stainless steel collar cannot move back, allowing water into the boat. One of the perennial complaints about PSS seals is that this has happened, whereas the reality is that the thing was never assembled properly in the first place. As has been said, using the supplied Allen key do it up as tight as you can manage. There is nothing that will harm by this, but don't try it using a metric key instead, which will turn inside the grub screw at relatively low torque. Then put the second grub screw in and do the same.

If ever you remove the screws make sure when replacing that the point is still good before replacing, so that the indent can take place.
Click to expand...

When I was an apprentice (many many years ago) we were always told to position the collar the drill an indent in the shaft before fitting the grub screw. With regard to the second locking grub screw I was alway concerned that the locking grub screw would damage the allen key hole in the first grub screw.

Cannot remember the type of grub screw point but IMHO they should be dog point not cone or cup point but a dog point will need a drilled indent where as a cone or cup point could be fitted without.

https://en.wikipedia.org/wiki/Set_screw#Point_types
 
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Rogershaw said:
Cannot remember the type of grub screw point but IMHO they should be dog point not cone or cup point but a dog point will need a drilled indent where as a cone or cup point could be fitted without.
Click to expand...

According to the fitting instructions they are in fact cupped points
 
Rogershaw said:
When I was an apprentice (many many years ago) we were always told to position the collar the drill an indent in the shaft before fitting the grub screw. With regard to the second locking grub screw I was alway concerned that the locking grub screw would damage the allen key hole in the first grub screw.

Cannot remember the type of grub screw point but IMHO they should be dog point not cone or cup point but a dog point will need a drilled indent where as a cone or cup point could be fitted without.

https://en.wikipedia.org/wiki/Set_screw#Point_types
Click to expand...

In practice it would be tricky but not impossible to drill a dimple for this application. AFAIR the instructions do not recommend it.
 
Rogershaw said:
When I was an apprentice (many many years ago) we were always told to position the collar the drill an indent in the shaft before fitting the grub screw. With regard to the second locking grub screw I was alway concerned that the locking grub screw would damage the allen key hole in the first grub screw.

Cannot remember the type of grub screw point but IMHO they should be dog point not cone or cup point but a dog point will need a drilled indent where as a cone or cup point could be fitted without.

https://en.wikipedia.org/wiki/Set_screw#Point_types
Click to expand...

I do drill a minute recess on the shaft for the cupped-point grub-screw provided by PSS - incidentally, always use their Allen key because the grub screws are Imperial sized.
 
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