6 ms·

Bolts and machine screws are very interesting. One mistake I, an amateur bike mechanic, made for years was not greasing bolts properly before assembly. It sou

by heads 3y ago

Bolts and machine screws are very interesting. One mistake I, an amateur bike mechanic, made for years was not greasing bolts properly before assembly. It sounds counterintuitive but without grease a bolt (or machine screw) will bind early with a high torque well before it is correctly tensioned lengthways. The torque is just a proxy for the tension and it is this tension which is needed to fasten your components together as intended. The grease means that when the torque to turn the bolt reaches the correct value then the bolt is also under the correct tension instead of being because it got stuck in the thread half way.

NegativeLatency 3y ago
I keep a toothbrush covered in grease for just this purpose. It’s also a magnetic one so I can stick it to stuff and it stays put.
- TaylorAlexander 3y ago
  > I keep a toothbrush covered in grease for just this purpose. Speaking from personal experience, this is also useful for brushing a robot's teeth.
- shmeeed 3y ago
  Where does one find a magnetic toothbrush?
  - NegativeLatency 3y ago
    It’s an old Sonicare brush head (a couple of fairly strong magnets are mounted to the bottom), there are also some kinda neat suction cup toothbrush holders you could use
myself248 3y ago
There are different torque-to-tension equations for lubricated and unlubricated fasteners, of various materials and platings.
WheatMillington 3y ago
Only grease fasteners if specified by the manufacturer, otherwise you're going to apply much higher than specified tension at the same torque value.
- Scoundreller 3y ago
  I need manufacturers to give "frame spent 3 decades in the salt belt, questionable metal compatibility and won't be taken apart until it breaks again" torque specs. It's like they think I'm going to take this thing apart and put it back together every year. j/k, finding torque specs for a decades old steel bike is a lost cause
  - serf 3y ago
    a good machinists handbook will give you torque specs that take into account fastener size, material, and thread-locking style.[0] mine is much older than linked version, but i doubt they'd revise that whole section out of it. [0]: https://www.amazon.com/Machinerys-Handbook-Toolbox-Erik-Oberg/dp/0831130911 https://www.amazon.com/Machinerys-Handbook-Toolbox-Erik-Ober...
  - nikau 3y ago
    In the car world that torque is measured in ugga duggas from the impact wrench
WalterBright 3y ago
Yes, that's the way aircraft fasteners are put on. There's another reason to grease them - the grease keeps the water out which prevents corrosion. In working on my cars, I always use a bit of grease when assembling fasteners. I've never had one come loose, nor have any rusted themselves on.
deelowe 3y ago
A properly designed service manual will specific if the fastener should be lubed. If it doesn't say lube/thread locker should be used, then the fastener should be torqued dry.
- loeg 3y ago
  In bicycle applications it is common to elide explicit instruction to use grease. Nevertheless it should be used.
johnwalkr 3y ago
It's a complex topic. Depending on the design, lubrication may be required or forbidden. And if it's required, it should normally be on the threads only, under the head is usually forbidden. And it's very important to understand that in most mechanical design with fasteners, the fasteners provide tension, and friction between 2 faces actually carries the load. Too little tension then fasteners can be sheared off. Too much, then the fasteners may not have remaining strength available for loads that act to add tension to the fasteners. Using a torque wrench to reach target tension is normally only about +/-30% accurate. Usually design margin allows for this but in very critical applications where margin is not feasible, calibration is done on a sample of the same materials, etc, or a more direct measurement done. More direct measurement can mean hollow fasteners that allow you to measure the amount of stretch, use of ultrasonic measurement to measure stretch, washers with integrated strain gauges, or cleverly designed "tension indicating" washers or fasteners. There are many types so I just those keywords for anyone interested. I am not saying that this design required such complex methods and sizing these fasteners should not be difficult. There is probably a mistake or lack of control in the assembly process.
- jacquesm 3y ago
  Lots of good stuff in this comment but I'm confused about the +/- 30% accuracy quote, in my experience it is relatively easy to reach sub 10% accuracy based on comparing different (good quality) torque wrenches against each other and against a calibrated one. Is there something I am missing here?
  - brettnak 3y ago
    The desire of fastener tension is clamping force or fastener stretch in some cases, not fastener torque. I think what the parent is saying is that a perfectly torqued fastener is only about 30% accurate in terms of clamping force.
  - nkurz 3y ago
    I presume his point is that you are measuring torque (potentially very accurately) but what you actually care about is tension. It's the variable factors affecting the relationship between torque and tension that results in most of the +/- 30% accuracy in tension, not the measurement of the torque.
ssl-3 3y ago
Torque specs only apply to clean and clear threads that are free of corrosion and grease -- unless otherwise-specified. Threads that are this way do not get stuck half-way. If the threads are already boogered up for whatever reason, then the torque specification is already wrong: The threads aren't clean, clear, and free of corrosion and grease. This can result in under-tensioned fasteners when using a torque wrench as a guide, since boogered threads (rather obviously) can present an impingement that allows a torque wrench to click off before proper tension is reached. However, grease is not a magical antidote to this condition. If the threads (boogered or not) are greased, then the torque spec is also wrong: Greased threads are also not clean and clear, and free of corrosion and grease. This can result in over-tensioned fasteners when using a torque wrench as a guide, since grease is (rather obviously) a lubricant -- allowing things to slide more-freely in a way that doesn't allow a torque wrench to click off until somewhere beyond ideal tension. --- So what do, then, as a home-gamer with a bicycle or maybe a car in the rust belt (but never an aircraft) full of dissimilar metals that are constantly rotting? You could perhaps kit up to do the Junker test such as in DIN 65151, and make a study of how different greases affects things. You can even make a career out of publishing your studies. Or: Just make sure the threads are clean, clear, and dry, and then assemble with an anti-seize paste which does not have lubricating qualities that affect final tension yield. (Permatex makes some, as do others.) --- (And in aircraft, always do what the engineers say. If the engineers are wrong, then: Stop doing whatever it is that you're doing, and consult them.)
stjohnswarts 3y ago
In some cases (some spark plugs are what I can think of) explicitly tell you NOT to grease them, as that will let you over-torque them and potentially damage threads (aluminum heads, steel plugs). So using grease is not always the recommended procedure.

by heads 3y ago

NegativeLatency 3y ago

I keep a toothbrush covered in grease for just this purpose. It’s also a magnetic one so I can stick it to stuff and it stays put.

TaylorAlexander 3y ago
> I keep a toothbrush covered in grease for just this purpose. Speaking from personal experience, this is also useful for brushing a robot's teeth.
shmeeed 3y ago
Where does one find a magnetic toothbrush?
- NegativeLatency 3y ago
  It’s an old Sonicare brush head (a couple of fairly strong magnets are mounted to the bottom), there are also some kinda neat suction cup toothbrush holders you could use

myself248 3y ago

There are different torque-to-tension equations for lubricated and unlubricated fasteners, of various materials and platings.

WheatMillington 3y ago

Only grease fasteners if specified by the manufacturer, otherwise you're going to apply much higher than specified tension at the same torque value.

Scoundreller 3y ago
I need manufacturers to give "frame spent 3 decades in the salt belt, questionable metal compatibility and won't be taken apart until it breaks again" torque specs. It's like they think I'm going to take this thing apart and put it back together every year. j/k, finding torque specs for a decades old steel bike is a lost cause
- serf 3y ago
  a good machinists handbook will give you torque specs that take into account fastener size, material, and thread-locking style.[0] mine is much older than linked version, but i doubt they'd revise that whole section out of it. [0]: https://www.amazon.com/Machinerys-Handbook-Toolbox-Erik-Oberg/dp/0831130911 https://www.amazon.com/Machinerys-Handbook-Toolbox-Erik-Ober...
- nikau 3y ago
  In the car world that torque is measured in ugga duggas from the impact wrench

WalterBright 3y ago

Yes, that's the way aircraft fasteners are put on. There's another reason to grease them - the grease keeps the water out which prevents corrosion. In working on my cars, I always use a bit of grease when assembling fasteners. I've never had one come loose, nor have any rusted themselves on.

deelowe 3y ago

A properly designed service manual will specific if the fastener should be lubed. If it doesn't say lube/thread locker should be used, then the fastener should be torqued dry.

loeg 3y ago
In bicycle applications it is common to elide explicit instruction to use grease. Nevertheless it should be used.

johnwalkr 3y ago

It's a complex topic. Depending on the design, lubrication may be required or forbidden. And if it's required, it should normally be on the threads only, under the head is usually forbidden. And it's very important to understand that in most mechanical design with fasteners, the fasteners provide tension, and friction between 2 faces actually carries the load. Too little tension then fasteners can be sheared off. Too much, then the fasteners may not have remaining strength available for loads that act to add tension to the fasteners. Using a torque wrench to reach target tension is normally only about +/-30% accurate. Usually design margin allows for this but in very critical applications where margin is not feasible, calibration is done on a sample of the same materials, etc, or a more direct measurement done. More direct measurement can mean hollow fasteners that allow you to measure the amount of stretch, use of ultrasonic measurement to measure stretch, washers with integrated strain gauges, or cleverly designed "tension indicating" washers or fasteners. There are many types so I just those keywords for anyone interested. I am not saying that this design required such complex methods and sizing these fasteners should not be difficult. There is probably a mistake or lack of control in the assembly process.

jacquesm 3y ago
Lots of good stuff in this comment but I'm confused about the +/- 30% accuracy quote, in my experience it is relatively easy to reach sub 10% accuracy based on comparing different (good quality) torque wrenches against each other and against a calibrated one. Is there something I am missing here?
- brettnak 3y ago
  The desire of fastener tension is clamping force or fastener stretch in some cases, not fastener torque. I think what the parent is saying is that a perfectly torqued fastener is only about 30% accurate in terms of clamping force.
- nkurz 3y ago
  I presume his point is that you are measuring torque (potentially very accurately) but what you actually care about is tension. It's the variable factors affecting the relationship between torque and tension that results in most of the +/- 30% accuracy in tension, not the measurement of the torque.

ssl-3 3y ago

Torque specs only apply to clean and clear threads that are free of corrosion and grease -- unless otherwise-specified. Threads that are this way do not get stuck half-way. If the threads are already boogered up for whatever reason, then the torque specification is already wrong: The threads aren't clean, clear, and free of corrosion and grease. This can result in under-tensioned fasteners when using a torque wrench as a guide, since boogered threads (rather obviously) can present an impingement that allows a torque wrench to click off before proper tension is reached. However, grease is not a magical antidote to this condition. If the threads (boogered or not) are greased, then the torque spec is also wrong: Greased threads are also not clean and clear, and free of corrosion and grease. This can result in over-tensioned fasteners when using a torque wrench as a guide, since grease is (rather obviously) a lubricant -- allowing things to slide more-freely in a way that doesn't allow a torque wrench to click off until somewhere beyond ideal tension. --- So what do, then, as a home-gamer with a bicycle or maybe a car in the rust belt (but never an aircraft) full of dissimilar metals that are constantly rotting? You could perhaps kit up to do the Junker test such as in DIN 65151, and make a study of how different greases affects things. You can even make a career out of publishing your studies. Or: Just make sure the threads are clean, clear, and dry, and then assemble with an anti-seize paste which does not have lubricating qualities that affect final tension yield. (Permatex makes some, as do others.) --- (And in aircraft, always do what the engineers say. If the engineers are wrong, then: Stop doing whatever it is that you're doing, and consult them.)

stjohnswarts 3y ago

In some cases (some spark plugs are what I can think of) explicitly tell you NOT to grease them, as that will let you over-torque them and potentially damage threads (aluminum heads, steel plugs). So using grease is not always the recommended procedure.