steering system design challenge

[whipper_snapper]

May I throw out an engineering challenge ?

The wheel on my 35' wharram cat is a big wooden thing mounted on a stainless steel shaft with a diameter of approx 15mm. The whole wheel/shaft assembly weighs perhaps 8Kg.

The shaft is mounted in 'bearings' which are simply holes through two thickish pieces of wood. This causes problems, mostly because the alignment is not good and the wheel wobbles. This in turn means that the loads from the autopilot are misaligned.

I would like to fix all of this by building a rigid stainless steel frame which we can bolt in place and which will hold front and back bearings for the shaft.

I can get the frame made-up here, if I can design it. But the the questions are:

1. What do I use for bearings and how do I mount them into the frame?
2. How should the frame be constructed

1. I wondered about simply machining bushes of teflon or similar and bolting them into place. Or something better like Vesconite ? How obtainable and machinable is something suitable ?

To keep weight and costs sensible, I don't want the frame to be too massive (it will have a width of around 1 foot and length of 18" to fit the space available), so the bearings will be sitting in holes in SS just a few mm (8mm??) thick. I worry about this putting big 'point loads' on the plastic, but I guess that a ring of small bolts through flanges which overlap will do it ? Any better ideas ? Do most boats use proper bearings with ball races ? If so how are these mounted and how do they fare in the marine environment ?

Any better ideas? What about a single bearing made of a length of a foot or so of suitable bearing material, a bit like a rudder shaft bearing ? Easier to keep straight and true but high friction ?

2. For ease of construction, the frame needs to be pretty substantial. I was thinking of making it from 8mm x 60mm bar or similar. That would give a minimal area for the bearing to sit on and be rigid. But how to assemble it ?There are people here who will weld or bend, but it must obviously be perfectly square, especially the faces must be perfectly parallel. Any suggestions on how to construct it ?

Thanks

 

[ianat182]

From previous threads on here the general consensus was for Delrin or Tufnol bearing material, both have good wear properties and Delrin has less water absortion than any Nylon spec;Tufnol does absorb water and allowances in thous are made for this when machined,and water also is a lubricant for the latter.(Never grease or oil Tufnol)
May I suggest that you post two or three general photographic views of your existing arrangement so we can relate this to your proposed solution and offer answers you may use.

ianat182

 

[misterg]

...But how to assemble it ?There are people here who will weld or bend, but it must obviously be perfectly square, especially the faces must be perfectly parallel. Any suggestions on how to construct it ?

Thanks

The normal solution (engineering wise) is to use plummer blocks or pillow blocks that allow bearings to be aligned on imperfect structures. You may have to hunt around to find them in stainless steel, though.

Andy

 

[bigwow]

I second the plummer box solution, used them on my Flica cat, not stainless but kept well greased.

 

[lw395]

I second the plummer box solution, used them on my Flica cat, not stainless but kept well greased.

You might be able to get them in plastic. no seriously!
I would want the design load to be along the lines of 'fat bloke hanging off rim of wheel'.

Or maybe a selfaligning bush in an alloy carrier?

 

[Salty John]

This type of bearing would be typical for your application:

One on each of your new cheek plates.

 

[Deleted member 36384]

The Wharram steering wheels normally control the rudders through a cable wrapped round the drum behind the wheel, which I assume is the same as yours. I think the pillow blocks are a bit OTT for what is a very basic and simple system.

I would not use stainless as it is likely to be unnecessarily heavy. You could make box structure out of 15 mm marine ply, triangular shaped, so that you have a wide base.

I would use sealed stainless ball bearing races suited to your shaft ID. They should be an interference fit on the shaft. Freeze the shaft in a freezer and the bearings should slip on.

If you cut the panels accurately to make your box you can pre drill the bearing holes in the ply. You may need to double up the ply thickness at the bearing holes. Assemble the box with the shaft in place and then Epoxy bond it together. This is more than accurate enough for alignment purposes.

You then need to make keeper plates for the bearings so that your shaft does not fall out. I would make stainless C plates such that the C opening is just bigger than the shaft ID. Drop the C plates over the shaft (4 in total), one on each side of the bearing and screw onto the ply.

You should now have a rigid structure with a bearing supported shaft running through it to mount. I'll leave the detail to you for the auto pilot mount.

I have been involved in the building of a 41' Phai "Captain Cook" and built my own 35' Tangarroa. I know that the above works better than the standard design. The marine ply is also easy to fix if it gets damaged, stainless is not. The Wharram's are not so high tech that a great deal of accuracy is required but you need to make sure that you mount the autohelm on the steering box.

I have also seen a tiller pilot mounted such that it steers the tiller link bar. The problem with the wheel, if you are using the standard design steering cable is that there is too much slack and the auto pilot is forever winding in slack. Direct auto helm steering on the tillers is better in my opinion.

Good luck and all the best.

Post Posting Edit: Do not use self aligning ball bearings as the steering box can be bonded together out of true.
There is no need to use them.

 

[whipper_snapper]

Thanks all, very helpful. I will do some Googling and follow-up on the suggestions.

edit: I had never heard of a plummer box, but they look ideal. I could simply bolt a pair of things like this

onto a flat block of hardwood.

I don't have good pictures, I will try to get some next week.

I am interested in the idea of putting an autohelm on the tiller. It is true that the present system using rope round the drum is very 'sloppy', but hand steering is actually very light and easy. However, the new Raymarine SPX5 really struggled on its first serious outing, for reasons I struggle to understand. However, when hand steering the 'wobble' in the system is not a problem, but I believe that much more accurate and true running of the wheel would help a lot.

Does anyone know if we can use the same computer and instrument head to power a tiller actuator. I really don't want to have to buy a whole new autopilot!! Doesn't the backlash in the rope/wheel system cause problems when steering from the tiller ?

I have an old picture that gives a rough idea:

and a picture that doesn't help at all, but shows the boat at anchor last week and I think she looks good! OldBoots - yes she is a Tangaroa Mk4

 

[Salty John]

That's plummer block, or pillow block, bearings.

 

[john_morris_uk]

Our wheel has needle bearings on each end of the shaft and these fit into machined in line bearing surfaces in the pedestal.

A hunt through a bearing catalogue should produce some sealed s/s bearing which you can incorporate into your design.

The bearings are cheap - why skimp on the feel and feedback of your steering system? Furthermore, the power consumption of the self steering will be improved by having as little friction as possible in the system.

Just because Wharram does it a particular way doesn't make it right. Wharrams are great - but built to a budget!

 

[Deleted member 36384]

.... Doesn't the backlash in the rope/wheel system cause problems when steering from the tiller.....

Not really, the wheel just flops about as the autohelm (Tiller Pilot) is more or less directly linked to the rudders but it could cause some high loads on the ram if the slop is big.

It is difficult to keep the rope tensioned as the hulls pivot anyway. Connect the steering ropes with carabiner at each end near the tillers. Between the carabiner and the steering rope (at each end) attach a small 3 or 4 part block and use this to maintain steering line tension. When the tiller pilot is connected simply un hook the wheel by removing the steering line at each end. Its easy on the Tangaroa and does away with the problems caused by pivoting hulls.

A tiller pilot will more than handle the steering and work with much less effort, which usually means greater reliability and a longer life.

 

[Avocet]

Could you cannibalise an old winch? The load case on a winch is very similar. You'd just have to bolt the winch base to your upright and then turn down the drum (they're usually hollow, so you'd probably have to make a thing that mimicked the INSIDE dimensions of the drum's bearing surfaces) and bore out the centre of the wheel to attach it. I don't know about other brands, but my winches have needle roller bearings in them and can handle massive radial loads - I doubt a wheel (even with a fat bloke hanging off it!) would bother it much!

 

[bigwow]

You might get some usefull info here

http://www.multihulls4us.com

 

[TQA]

reduce reuse recycle

How about a motor cycle headstock / handlebars / front fork set up from a scrap yard. Will cope with a fat bloke hanging off the wheel. Work out how to attach wheel and drum and you could have an easy solution.

 

[whipper_snapper]

Not really, the wheel just flops about as the autohelm (Tiller Pilot) is more or less directly linked to the rudders but it could cause some high loads on the ram if the slop is big.

It is difficult to keep the rope tensioned as the hulls pivot anyway. Connect the steering ropes with carabiner at each end near the tillers. Between the carabiner and the steering rope (at each end) attach a small 3 or 4 part block and use this to maintain steering line tension. When the tiller pilot is connected simply un hook the wheel by removing the steering line at each end. Its easy on the Tangaroa and does away with the problems caused by pivoting hulls.

A tiller pilot will more than handle the steering and work with much less effort, which usually means greater reliability and a longer life.

Sounds like a good solution and suitably Wharram!

I guess the big disadvantage of a tiller drive is you loose the ability to make an instant hand over between autopilot and hand steering by wheel. But we could live with that.

The only real problem I see is the connection between actuator and tiller. Because of the geometry of the steering, the tiller describes a complex movement when it swings. To get the linear rudder reference sensor to work we had to improvise an elaborate 3-axis universal joint. But it would have to be very solid to transmit drive - I guess some kind of ball and socket join?

 

[whipper_snapper]

Just because Wharram does it a particular way doesn't make it right. Wharrams are great - but built to a budget!

Absolutely, and regardless of whether or not I switch to a tiller autopilot system, I will sort the wheel bearings.

In fact, the story of this boat is a basic sound boat, but with every fitting, horribly under-engineered. So we have systematically gone through everything, gas, electrics, standing rigging, running rigging, fundamental things like hull/deck attachments etc etc and replaced everything with over-engineered solutions! Still need to sort-out engine and sails! But it is getting to be a nice boat one day.

 

[rogerthebodger]

Stainless steel pillow block bearings

http://za.rs-online.com/mobile/search/searchBrowseAction.html?method=getProduct&R=2328625

 

[whipper_snapper]

Stainless steel pillow block bearings

http://za.rs-online.com/mobile/search/searchBrowseAction.html?method=getProduct&R=2328625

Thanks, now that I know such things exist and I can abandon my over-elaborate stainless-steel frame idea, it should be easy. In fact, I can almost certainly find something suitable in Nairobi's industrial area.

For fitting, I hope I can find a bearing with a true interference fit to the shaft. I can cool the shaft in liquid nitrogen, which should make it a sliding fit and then I hope no other attachment will be needed to hold it in place.

 

[rogerthebodger]

Most pillow blocks have grub screws in the inner race that can be used to locate the shaft in position some I have seen have a tapered collet arrangement but there are used when the thrust loading is quite high.

I have used a flanged "pillow block to take the thrust on my prop shaft with taperlocks and each side to resist the prop axial load in forward and reverse, but and axial loading in your setup will be quite low so grub screws should be OK.

 

[TonyMS]

Wheel

I had a similar problem. Here are a picture of one of the wheels I made and a diagram showing how it is made.

The wheel hubs are made up as a double-decker aluminium sandwich, with
fillings of plastic balls. The wheel rims are 19mm tube, bent progressively
through rollers. The cables are 4mm Dyneema.