Measuring fit of a taper using blue

[Machina]

Hello all,

I'm working on a job where I am assembling a hub to a shaft. This is an interference fit onto a taper (will be using hydraulics) but I have to check that there's sufficient contact area between the items before I drive it on (the spec. calls for 85%).

So I'm trying to measure this using engineer's blue but I'm not certain I'm getting a reliable result.

Here's a basic description of what I've tried (tried some variations)
1. Clean the inside of the hub.
2. Clean the outside of the shaft.
3. Coat the inside of the hub with a thin coating of blue (using Dykem Hi-Spot).
4. As carefully as possible lightly mount the hub to the shaft.
5. Fit hub locknut.
6. By hand turn the locknut up against the hub (trying to make sure the hub is square).
7. Remove locknut.
8. Use some wooden wedges to loosen the hub from the shaft.
9. As carefully as possible remove the hub from the shaft.
10. Examine the shaft to see where blue has transferred.

All I'm getting is a (fairly wide) ring around the largest diameter of the taper and a single patch that covers about 1/6 of the remainder of the joint, that patch stretching the length of the joint.

It may be that the fit is no good, but I need to be sure.

It's been suggested that I need to turn the hub slightly on the shaft but the taper is such that it locks up and prevents rotation, unless it's quite really quite loose.

The length of the joint is relatively short, about 135mm. Average diameter of the fit is about 179mm. Taper is 1:24 (included angle 2deg 23min 13sec). So I'm concerned that the hub may be tilting slightly on the shaft and giving me a false indication.

I'm sorry for the long post but I've tried to explain it as clearly as I can because I'm pressed for time. If there's anything I've missed then please ask any questions. Any comments or suggestions would be greatly appreciated.

 

[1gibson]

The spec calls for 85%, or the spec calls for the design to be 85%?

I would be interested to see someone come up with a definitive method to "test" this, but you might re-read the spec before you pull all of your hair out.

 

[JJPellin]

For a 1/2 inch per foot taper designed for hydraulic mounting, I would require 90 percent contact verified at each installation. 85 percent seems quite reasonable. Keep in mind that interference is the only drive. No key.

It is much better to use a set of ring and plug gauges to check contact. This can verify the taper even beyond the contact area.

If you must use the actual hub to the shaft, I would suggest a few things. The parts must be perfectly clean and oil free. I use electro-contactor cleaner. I prefer to use Prussian Blue. Your method sounds good. Once the hub is fully seated, I give it a sharp rap with a soft hammer. It can be difficult to get off. But it must be fully seated.

If you need to lap to improve the fit, you cannot lap the hub to the shaft. You will need lapping tools. If you lap the hub to the shaft, you will grind steps into both parts.

Johnny Pellin

 

[byrdj]

that sounds like the procedure used for putting the coupling on a deLaval Boiler Feed Pump shaft. I don't ever remember being ask to evaluate a fit that didn't meet the 85% blue check.

You might try having the hub suspended of of a sling and taping in a "star" pattern to "walk" the hub on as far as it can without stretching it.

You need to be able to get a consistant blue contact since that position is used to calculate your "pull up" to strecth the hub for the press fit. You should be measuring "stand off" for each attempt to blue to see if it does show a poor contact

 

[Tmoose]

Dykem is pretty thick, compared to Prussian blue. Dealing with the pale color of a thin coat of prussion blue can be challenging.
is it possible to test with the shaft vertical?

Is there an installation spec for a certain amount of "draw up" of the hub along the shaft? If that results in 0.001 inch or more diametral interference then a few tenths difference in diameter at the ends of the taper are not as significant as some specs would indicate.
Turning ("Wringing" ) the hub on the shaft is a common technique.

Can you detect the hub rocking when first set on the (vertical) shaft?

 

[GregLocock]

I suspect you'll get an alarming diference ig you use Plastigauge to check the no load conformity of the two tapers, vs a 'sharp rap' method vs the almost impossible to measure full installed method.

As you tighten the nut on the taper the spike lengthens, and the cone compresses, thereby altering the basic shape of each component in service. This may not matter in your context but it is certainly an issue in some cases.

I'd have a chat to the customer or designer to figure out what the 85% refers to.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[JJPellin]

Please be careful about suggesting that the OP can accept lesser contact. Without knowing the speed, torque and pull-up, this is risky. This is a very large shaft. I have seen hydraulic fit hubs come loose on the run. 4000 horsepower, 9000 rpm. This happened with 0.003 inch interference and 90 percent contact. And lapping a hub to a shaft and then pulling it up a taper guarantees that the resulting contact will be nearly zero.

Johnny Pellin

 

[JJPellin]

Blue check verification of taper contact is a common requirement for large turbo-machinery. The lowest contact requirement I have seen is 75%. For hydraulic mounted hubs, 85% or 90% is common. Deviation from this requirement could result in catestrophic failure.

Johnny Pellin

 

[Machina]

My current thinking is to make up some tooling (probably just bits of plate and studding) to help get the hub off the shaft and then try the blue check again using a soft faced mallet to tap it onto the shaft.

Other ways I could try and push it on include tightening the locknut a little or put the hydraulic nut on and use a very low pressure. But I think tapping it will square it up more initially.

1gibson
85% requirement - the actual contact area must be at least 85% of the theoretically available contact area.

JJPellin
cleaning - I'm using isopropyl alcohol to clean the parts each time.

gauges - yes, I'd much rather have them but don't at the moment. If it gets to the stage of requiring lapping then I'll have to get something. Although I'm not certain what. If I lap hub to one gauge and shaft to another, how do I know the gauges aren't being significantly worn? Or does that not matter if I check the real parts against each other again?

hammer - I've been reluctant to do so because of the difficulty in getting it off but I think I'll make a few bits to act as a puller to help get it off.

byrdj
sling - to get the hub onto the shaft I'm using a sling but I haven't tried tapping it on yet, well, only wih the flat of my hand.

stand off - yes, I'd like to have measured it each time but didn't have any slip or enough feeler gauges to measure the gap to the next component accurately. I had used a big bunch of feeler gauges to try and makes sure the gap was consistent at various points, but they were quite worn (buckled and bent). I've got a new set of feeler gauges now and will try.

Tmoose
Dykem - yes, it is pretty thick, but it's what the engineer in charge has requested. I have thought about thinning it down a bit. Is it possible that too thick a layer is forming a high spot by itself in one place? I'd've thought the paste was soft enough to move around. Might give it a try with thinned down Dykem.

Draw up (drive) - Yes, I've got a number for how far onto the shaft the hub should go but I've got to meet both parts of the specification.

Turning ("Wringing") - The taper locks up pretty much as soon as I get the hub onto shaft and then I can't turn it.

Vertical - The horizontally mounted shaft is in the machine still (shaft is approx. 1500kg, machine much more). There's no chance I can move it. Which is a pity.

GregLocock
Plastigauge - I've used such for measuring clearances but I'm looking for contact here. I guess I could assemble it and argue that if I get equal clearance then all areas would contact at the same time. But I think that might be even more difficult to line up.

JJPellin
"lapping a hub to a shaft" - I assume you mean using lapping hub to tooling/gauge, and then shaft to tooling/gauge as you suggested earlier?

Requirements - As well as what you've said, on this job I don't have the authority to change the specification so will have to keep trying.

 

[JJPellin]

Normally the gauges would come as a double set. One set is for checking only. You would check the hub to the plug and the shaft to the ring. Then you would verify the contact between the ring and plug. The other parts of the set are lapping tools. The lapping tools are cast iron. The small amount of lapping you are likely to need should not significantly affect the lapping tools. But, the final proof is always to go back to the checking tools to verify the result.

Excessively thick blue can hold the parts up and affect the results. The application of blue needs to be very uniform. I have had one other problem with the blue. If it has ever been frozen or if it is old, it might feel gritty between your fingers. If it feels this way, throw it out. It will give you a false low indication for contact.

The main problem with a hub this large is the weight of it. A similar one that I have mounted weighed about 300 pounds. This makes the check very challenging. This is another reason for the use of gauges which are much lighter.

We usually measure stand-off with a depth micrometer from the end of the shaft to the face of the hub. Depending on how you do the rigging, this may not be possible during the blue check.


Johnny Pellin

 

[Machina]

The blue I'm using is a brand new tube, so I'm happy that that's OK but it is thick and I think I'll try thinning it down a bit.

The weight of this one is not as bad as the situation you had. The hub is about 80lbs. We could lift it manually into position but we've been using a sling to try and control the location of the hub as we slide it over the shaft, trying to avoid smearing blue everywhere.

In this case the other measurement I'm taking is between the inboard face of the hub and the next component along. We're measuring this in order to make sure we've got enough space to drive the hub far enough on to get the desired interference.

 

[JJPellin]

Coincidentally, I am working on a final plan today for our upcoming turnaround. The first task I came to this morning is the removal and mounting of the coupling hubs. These hubs are also hydraulically mounted, but much smaller than yours (about 75 mm bore). The coupling manufacturer's procedure specifically calls out a required minimum contact of 85%. Our internal specifications will override this. We will still require 90% contact. I just received verification from our millwright foreman that we have both ring and plug gauges and lapping tools for these tapers.

I would probably not thin the blue by diluting it. But, take your time and apply it thinly and evenly.


Johnny Pellin

 

[Tmoose]

When the OP said he must use Dykem, this layout fluid is the stuff I pictured him using.

http://www.grainger.com/Grainger/DYKEM-Steel-Blue-Layout-Fluid-2C945?Pid=search

it dries to a hard opaque finish quickly, and is at least several ten thousands (0.0001s) of an inch thick. Applied by a brush the thickness will be somewhat variable. That makes no difference when used to mark layouts on components. I think There will be a tendency to run to the underside of the shaft and thus be quite thick on the underside. It can be sort of wiped off by mechanical contact but comes off in strips almost like paint and can even jam up between components in contact.

When looking for a soft material transfer between components this other "blue" product is sometimes used.

http://www.grainger.com/Grainger/PERMATEX-Prussian-Blue-3DPP9

It does not dry, but in my limited experience is mighty transparent when thin so is very hard to see.

 

[JJPellin]

Tmoose, The product the OP is describing is made by the same company as the dry lay-out blue. But, it is a non-hardening paste like Prussian Blue. The Hi-Spot blue is very similar to the Prussian. Reference the path below. You are absolutely correct that the layout blue would not be suitable for this task.

http://www.grainger.com/Grainger/ww...&Ntt=hi-spot&N=0&GlobalSearch=true&sst=subset

Johnny Pellin

 

[Machina]

It seems Dykem make a few products, and are better known for marking out blue but the one I'm using (Hi-Spot Blue) is, as JJPellin describes, non-hardening. It is quite thick and difficult to get a thin layer on, so I'm wondering if I'm building my own high spots with the blue.

It does have a strong colour, but I'm also wrapping a piece of clean white paper around the after checking the fit in order to make a record of the measurement so I'd expect faint traces to show up more clearly on that.

I'm still not getting a good result, tapping the hub on using a soft faced mallet, so am considering tightening the locknut a little whilst tapping. However I'll have to sort out a puller to get the hub off again. I'd planned to do that anyway but didn't have a chance to.