[lagfish]

Is there a special kind of nut you use for this, and what's the advantage of using it versus a simple hex nut?
Does it make a difference to use a fine thread or coarse thread? Thanks.

[holbieone]

any nut will do

read these articles

http://www.nhbb.com/precision/engineering/preload.htm
http://www.bearings.machinedesign.co...emech6_43.aspx
http://www.bardenbearings.co.uk/index.cfm?articleid=717

[philbur]

Threaded nuts are not really the correct way to preload bearings. However I guess we all use them. The finer the thread the fine the adjustment of the preload. Also using a second nut to lock the first will usually upset the adjustment of the first and is not the best method. The nut thread and faces should be lathe cut as should the shaft thread.

Is there a special kind of nut you use for this, and what's the advantage of using it versus a simple hex nut?
Does it make a difference to use a fine thread or coarse thread? Thanks.

[lagfish]

Why is using a nut wrong? Is this because the nut is not square to the face of the bearing? What if I used a square spacer between the nut and the bearing?

[M100]

Why would a nut *not* be square to the bearing?

[philbur]

Why would it be square. If the thread is lathe cut, yes, if you drill a hole and use a tap almost certainly not.

Phil

Why would a nut *not* be square to the bearing?

[philbur]

Even a fine thread is quite course for fine adjustments of load. Also how will you actually know what the preload is? Forget a torque wrench, you would have to know the frictional resistance to rotation between the two thread halves.

Using a nut isn't wrong. it isn't the correct way, that's not the same thing. Setting the preload to high shortens the service life of the bearing. In a worst case this could be from 10 years to 10 minutes or more probably in a hobby environment from 50 years to 20 years. So careful use of a nut, although not the correct method, can still produce an acceptable result.

Phil

Why is using a nut wrong? Is this because the nut is not square to the face of the bearing? What if I used a square spacer between the nut and the bearing?

[lagfish]

hey, even if it gets me just 1 year of service, I'm happy. I can just replace the bearings for $13 a pair in 10 min, rather than spending days preloading high quality bearings costing many times that amount.

Can you also not experimentally determine the preload? For example:
Get a precision spring of a known K value, and place it in between the two bearings you're using and on the same ballscrew.
Using the K value figure out how much deflection of the spring you need for the preload you want.
Tighten the nut with a torque wrench until the desired deflection on the spring is reached.
Note the amount of torque needed, and this should be the amount of torque needed to preload it on the real bearing block

[CarbideBob]

Bering nuts are ground square to the threads on their contact faces.
A precision bearing pair has the preload ground into them. No mater how tight you tighten you get the same preload.
On all tapered roller setups and some non-precision angular contact setups preload is controlled by how tight you screw it down. Consider drilling a hole through the side of the nut and putting in a small set screw to lock it on the shaft (brass tipped set screws work great for this). This will give you better control over the preload than using 2 nuts.
Bob

[lagfish]

where can I buy square ground bearing nuts for metric threads (12mm) on the internet?

I guess I can grind them myself, but that takes a lot of work.

[holbieone]

hey, even if it gets me just 1 year of service, I'm happy. I can just replace the bearings for $13 a pair in 10 min, rather than spending days preloading high quality bearings costing many times that amount.

Can you also not experimentally determine the preload? For example:
Get a precision spring of a known K value, and place it in between the two bearings you're using and on the same ballscrew.
Using the K value figure out how much deflection of the spring you need for the preload you want.
Tighten the nut with a torque wrench until the desired deflection on the spring is reached.
Note the amount of torque needed, and this should be the amount of torque needed to preload it on the real bearing block

with that quality of bearing your thinking a little over kill

as long as the bearings run smooth you should be ok

[holbieone]

if your shaft being preloaded does not heat up then all you have to do is preload the bearings just till there's no end play

if the shaft heats up and the bearings loosen with the thermal growth of the shaft then you will have to tighten the preload up so at that temperature there is no end play

if the shaft heats up and the bearings tighten with the thermal growth then you will have to loosen the preload so there is just no end play at that temperature

[M100]

Why would it be square. If the thread is lathe cut, yes, if you drill a hole and use a tap almost certainly not.

Phil

It was intended as a rhetorical question, but in the circumstances I would have expected anyone concerned about bearing preload would have a lathe or, if they were in the back end of nowhere and latheless, have the skill to hand tap accurately.

At the end of the day, this is "benchtop mills" after all, off the shelf commercial nuts with off the shelf commercial washers will almost certainly suffice as ground end nuts will make 3/5 of 5/10th of bugger all difference

But given the choice I'd go for two slotted nuts (google DIN 1804) - Maryland Metrics http://mdmetric.com http://mdmetric.com/fastindx/ud52_54.pdf is a US supplier but they should be available from any decent bearing supplier.

[philbur]

Not really, 1/30th or less of a turn could easily be too much so you would be trying to measure very small differences in deflection. With all the trouble necessary you may as well size and make a precision spacer.

For the class of work you are doing use a standard nut, drill and tap a hole for a locking screw as per CarbideBobs suggestion. Then tighten the nut until you can measure no free play with a DTI and a hard push/pull. Check that the bearing runs freely then tighten the locking screw and you are done. Finally use the DTI to check for free play under operating load. If detected tighten the nut a touch further and recheck under load.

Is this a precision set-up, no? Is it more than good enough for your purposes, yes? Some would be unhappy and call it a compromise. They fail to understand that all engineering is a compromise. Good engineering is what gets the job done for the least time, effort and cost, anything else is over-engineered.

Phil

PS: I admit to being prone to over-engineer, but I'm constantly working to correct this.

Can you also not experimentally determine the preload? For example:
Get a precision spring of a known K value, and place it in between the two bearings you're using and on the same ballscrew.
Using the K value figure out how much deflection of the spring you need for the preload you want.
Tighten the nut with a torque wrench until the desired deflection on the spring is reached.
Note the amount of torque needed, and this should be the amount of torque needed to preload it on the real bearing block

[lagfish]

Thanks everybody for your input