Static balance gadget

I want to balance a 16inch diameter 3 bladed propeller from a launch. I can only do a static balance as I have no equipment to do any other sort of balancing. I guess it weights several Lbs

I intend to take the prop off the boat & turn up a temporary shaft/ mandrel about 400mm long. It will be 25mm with a taper to take the prop so I expect one end will be about 18mm diam. I will not need to thread it if the taper is a fairly good fit. I have not done such a taper before but will search youtube for guidance. Does not need a key

To do the balance I want to make a frame with 4 wheels. 2 overlapping each end. Each one on bearings & as free running as possible. I reckon about 3mm th & 100-125 mm diameter.That should be easy enough on my Warco 240MV lathe

What I am not sure is how far apart to put the centre of the pairs. If they are close together such that the tangents are fairly horizontal they may not rotate. if they are too far apart with the tangents almost vertical they will put a lot of side pressure on the wheels & increase friction then just jam.

So there must be an optimum angle for the 2 wheels to meet so that I get a good friction free rotation to balance the prop.(I think)

What is it?

Does anyone have any experience of static balancing & is this infact the right way to balance the propeller. I know this is an engineering forum but presumably some forumites have had to balance equipment at some time in their careers

I guess that in the water it rotates at max 3000 RPM. At tickover revs (800RPM) it shakes the bearing violently, but begins to run smooth once the engine revs pick up. I expect this is because the diesel engine runs smoother plus the water has an effect as well

Edited By Sam Longley 1 on 28/07/2020 17:25:00

Edited By Sam Longley 1 on 28/07/2020 17:32:46

The next question to the above is - If the wheels are 3mm th I cannot just mount ball races into 3mm steel discs. So presumably I need to fit a boss to each disc because it would be too much steel to turn . So what is the best way to fit a boss & then get absolute concetricity on 100-125 diameter & press fit the bearings.( Warco 250MVlathe) The bearings can be any diameter but I would expect something to fit 10mm bolts so I can fit them to the frame

I cannot fit the bosses & bearings & turn the final radius on a mandrel because the bearings would just spin. But i need to make sure that the wheels are true to the bearings. I am assuming ball race bearings or would something else be OK, ie would oilite bushes run freely enough

Edited By Sam Longley 1 on 28/07/2020 17:35:37

Use a Jones and Shipman grinding wheel 'knife edge' rotating balance assembly. But if you want to build one research how they did theirs. Discs need to be about 6" to 8" diameter with an incidence angle between bearing centres and prop centreline of roughly 60 degrees.

Has to be mounted true and level (so put some levelling adjusters on the frame), spin the prop assembly up and wait for it to come to rest, not if it 'rolls back', marke the lowest point and repeat several times. If all is well and it consistently stops in the same place then add some weight on the opposite side (Blu Tac is useful for this). Repeat and check again, more weight if it still stops at the same point, less if it stops with the weights 'down'.

It's a long tedious process but you'll soon get the hang of it - if not, sit down and have a cup of tea, and have another go!

Sam, isn't this a bit complicated? What if you mounted the prop on a dummy shaft projecting from each end of the hub, say18mm dia. both sides. You could make up a fairly simple pair of elevated rails, say 30" long and allow the prop and shaft to run along them. this would enable you to locate the heaviest point of the prop. prior to metal removal or whatever to improve balance. Or am I missing something?

The way se used to statically balance fan impellers was to mount them on a mandrel, place on a pair of knifedge rails and note where the impeller comes to rest, correct by trial and error with lumps of putty/bluetack, when in balance weigh your putty and permanently fix metal weights (or remove metal at 180 degrees if there is enough)

Now use fancy electronic machines of course but static balance is good enough for narrow components unless you have a tight spec to work to.

Of course the knife edges have to be level to ensure accuracy.

Bill D.

I made the ends of my grinding wheel balance arbour the size of the largest bearings I had hanging around, IIRC they were about 15mm ID/30mm OD

I flushed out the bearings with wd40 to remove all the grease and then set them into v blocks.
That is sensitive enough to detect a a small out of balance on an 8” wheel, such that the spindle weights moved together make it rotate quite quickly.

Can you make the ends of your arbour small and do a similar thing?

Dave

Whilst not the sme weight one old fashioned trick for balancing a pair of model heli blades was to mount them opposite each other on a central mandrel and balance the mandrel ends on ice cubes on a pair of risers - very low friction and very low tech but 700mm heli blades at 2000rpm benefit from balance.

(In the case of the heli blades we also balanced them so they teetered at the same outboard point as each other)

pgk

Probably stating the absolutely obvious, if straight knife edge rails are to be used to check the balance, they must be absolutely level, in both planes. If they are not, the prop on its arbor will just run down the slope anyway, and keep running until it reaches the end, or falls off.

On a level rail, the prop will turn, rolling on the arbor until the heaviest point is at the bottom.

The easiest way to obtain static balance would be to remove metal from the "heavy" blade, until in balance.

Being a propellor, drilling holes into the blade would affect performance, and drilling holes in the boss might require a lot of metal to be removed. Unless wildly out of balance, a little judicious filing of the OD might do what is required.

Metal removal at each trial should be minimal You do not want to change a wobbly four leg chair into a very low stool ! Little and often is good motto.

Howard

Actually that seems to be a good idea

Compared to the other option it does not need one to align & level 2 rails. With the rails I was worried about them being inverted "V" shaped & dead straight as well as in line & perfectly level.

To balance the prop I was going to gently abrade/grind the surface on the back of the heaviest blade.

Edited By Sam Longley 1 on 28/07/2020 20:41:11

Some years ago i had a problem with the props on a Fairline targa 33 i had. They were stainless ones. It turned out they were not out of balance , but just slightly bent. Which was causing the vibration. Once trued up they were all good. Good luck with the project.

Steve.