alignment gap and sag

Hi! If you are using a correct coupling for your equipment, the coupling data should show you the limitation tolerance in terms of alignment, i.e. axial, angular & lateral

Regards,

Jojie_oak

What is the shaft diameter?

How much shaft overhang is there? i.e. the distance from the coupling to the closest bearing.

What is the mass of the coupling?

I am asking the questions to see if you have shaft droop which will often cause alignment problems and vibration.

can u please tell me some thing about shaft droop.

my coupling diameter is 500 mm.

distance from first bearing support is 30 mm.

its a rigid coupling.

gear box is rigidly mounted.

and diesel engine is on the mounts with vulkan coupling in between engine and gear box.

distance from first bearing support is 30 mm. ? that is small 30 mm = .030 m = 3 cm

If shaft overhang (distance from coupling end of the shaft to first support bearing), and the coupling mass is too great shaft droop or sagging will occur, or in other words it will bend.

The mass of the shaft overhang = 0,7854d²l x ρ (density) + mass of coupling

If this total mass is is too much, compared to the diameter of the shaft, the shaft will sag or droop, making it almost impossible to align the output flange of the gearbox to the companion flange on the shaft, if it is a rigid coupling.

To check for shaft droop:-

1. You have calculated the mass of the shaft and coupling, put a chain or sling around the companion flange and secure it, directly above the coupling via a spring balance and turn buckle. Set the spring balance to zero (until just take the mass of the shaft) then keeping an eye on the spring balance ,tighten the turn buckle until you take half of the calculated mass. Then carry out the alignment, providing other necessary factors have been fulfilled.

2. Place a dial indicator on the top of the shaft as close as possible to the bearing and a second dial indicator on the end flange of the companion flange (or the closest concentric machined surface on the coupling). Set both dial indicators to zero the place a jack under the shaft and adjust it until it just touches the shaft. Jack the shaft up until the dial indicator on the shaft at the bearing end moves 0,001"

Check the reading on the second dial indicator on the coupling, then slowly lower the jack to half the reading. the axis of the shaft is now on its correct centre then proceed with alignment, if other major factors are fulfilled.

I have found that magnetic blocks secured to the bearing housing and the output flange on the gearbox are as good as any points to secure the magnetic blocks.

It may be of interest to note that this not being carried out is one of the major causes of vibration and the problems associated with it in fishing vessels in Australia.

i got about speed droop.

one area for concern at the moment is when i am taking sag with dial indicator i am not able to achieve the value i have set on top. i.e, i have set 4oo mm on top and i got readings 402 at 90 deg, 404 at 270 deg and 403 at 180 deg and coming back to top i should achieve again 400 mm but iam not able get the same, iam getting somtimes 402 or 403 not fixed all the time. please what can the cause. in my view first i thought dial gauges, i have changed the one an got the same readings. next what clicks is it may be due to backlash of gear. please send some comments.

"next what clicks is it may be due to backlash of gear"

You are not fixing the dial indicator to the gear box flange and turning the gear box flange around the companion flange are you?

Are these readings that you have taken are they of the rotation of the companion flange (the flange on the shaft)?

Is the distance from the end of the shaft to the first bearing only 30mm, only this seems unusually small, very small??

What are your face runout readings, (your feeler gauge readings between the flange faces?

yes i am fixing my dial indicator stand on gear box flange and dial indicator on shaft flange. by rotating gear box i am taking reading on shaft flange. even though a matter what clicks me is also direction of rotation. my gear box should rotate in ccw direction and i am taking readings on cw direction.

yes give me some time, the reading for the distance from the end of shaft to the first bearing is not 30 mm. i will come back to u.

about face runout readings, i have taken axial gap with the help of feeler gauge and the readings are as follows. top 90 deg 0.45, bottom 270 deg 0.4, right 0 deg 0.45 and left 180 deg it is also 0.45 mm. this has been set because requirement was to maintain axial gap of 0.3 to 0.5 mm and angular distance 0.02 at 100 mm, my gear box coupling dia is approx 490 mm.

May I suggest that you affix your dial indicator to a static area and turn your shaft.

If this is possible, let me know your results. Taking them as you have gives no real indication of your shaft and companion flange. because you have movement in the output shaft of the gear box due to bearing clearances etc.

It could be really difficult to specify what will happen during a dynamic running of a machine with static readings,

What happens when the gearbox is at running temperature? does the output shaft move towards the coupled shaft in that condition? or once the unit is to operating temperature, you have lost 50% of the end float (diagram above)

Is the Prime-mover (whatever is powering the gearbox) solid mounted to restrict movement over the rev-range? do your readings change when there is load placed on the shaft/gearbox?

Is the shaft your trying to couple the input or out to/from that gearbox?

Giving more information gives greater understanding to who is trying to help you ;o)

Sounds like you have no universal joints where shaft meets gearbox. If you do put one in .. it will save you alot of wear and tear. Otherwise without univ joint you will have to make sure its exactly lined up or it will wear out the splines in the connection. on the shaft usually requiring replacement shafts.. We went thru this with one system and it wasn't fun.

Quite often you have to machine one of the faces on a universal joint at an angle of 2-3⁰ so that the joint moves enough during power transmission otherwise you can stuff it up in very short time.........been there............ done that.

In my area of operation there is only one port where they have universal joints or couplings and there's about 20 cray boats operate from it, it's called Carpenter Rocks and this place would put the Beverly Hillbillies to shame. No one else in a dozen or more ports will have anything to do with them.

When you have a reasonable sized prime mover, one of you best checks after shaft alignments carried out is carry out a crankshaft deflection test, even with smaller engines, personally, I prefer Flexibly mounted engines and decent either rigid or, preferably flexible couplings that must be set up correctly.

Which face of the uni gets machined?

Have you tried going for CV joints instead of uni joints?

I don't think it matters which face, but usually the face that couples up to the output flange of the gearbox.

Personally, I have not tried cv joints, and I cannot recall any fisherman that has on their vessel. and over the past 27 years since I have been involved with them,I have come to know quite a few, and that is Australia wide.

Whats the machining angle your talking about on the face of the Uni joint?

I couldn't find what I was looking for Snaketails, but if you imagine in the top photo the RH end of the universal , instead of having a cardan shaft had a flanged end, the same as the LH end, you have it.

In the vessels that had this arrangement the gear box and shafts were not in perfect alignment, but good enough to cause too little movement in the universal, thereby causing very swift failure due to lack of lubrication.

By machining the flange faces 2-3⁰ movement and hence good lubrication of all the needle bearings is achieved.

Post the Drawing.

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