Engineering help please - flex in stainless tube and motor power required for spit

[19brendan81]

Hey guys - im builing a spit roaster and have a piece of 25mm x 1.6mm stainless tube that I want to use as the bar. It will be supported at either end, and will stretch over a 1700mm span. Can anyone tell me how far this will flex with a 30 kilo pig on it? can you do the same for a 40 kilo beast too?

Also, how many watts would be required to turn a 40kilo pig at 8 revs per minute?

Thanks in advance,

Brendan

[simonl]

Hi Brendan,

Personally I would find out the flex by simply supporting the stainless rod at each end (as intended) and hang a bucket of water (or 2) in the middle. Then again it has just occured to me that maybe you are intending to purchase the tube and wish to know whether it's suitable BEFORE purchase? In this case I would have a look in a store like Bunnings or BBQ's galore at their spit kits and look and their sized rods and the rated capacities in Kg, then double it for your own use as a guide.

As for the HP, a properly balance beast won't require a huge amount of HP. You can buy counter balances to better balance the meat after its been put on. More HP will be needed if out of balance but I think 150W would be almost overkill once it's geared down to 8 rpm.

While nowhere near as big as your intended design, I have a portable spit which I take camping. I made the skewer out of 12mm sq stainless and the motor runs on 2 D cells and I can easily put 10Kg on it.

I'll be interested to what your progress on this. I may have to copy your design later.... :q

Simon

[19brendan81]

Im almost certain it will fine....I wanted something at least 2mm thickness but the store didnt have it. I was using some online calculators to try estimate deflection but was getting ridiculous results....so was hoping someone on here could confirm it for me.

re motor...yeah pigs are always way out of balance...so are lambs (ive swung a few of each before). I have an old washing machine motor that I plan on grafting to my old spit roast motor (that got left outside for some months and ruined). The old motor is burnt out but the reduction gearbox is fine. I am slightly concerned that the washing maching motor will be too powerful for the gearbox as the input shaft for the gearbox is only about 8mm....we will see. I will be filthy if the shaft shears halfway through a roast...but I doubt that would happen unless the pig jammed up for some reason.

[Ueee]

Hi Brendan,
Hope you and the new family member are well.

RC posted a pic of the maths for sag in a test bar in the scraping thread, not sure if that will be any use for something supported at each end though.....
http://www.woodworkforums.com/f65/scraping-projects-142774/index19.html#post1693142

Cheers,
Ew

[simonl]

Washing machine motor? Is there anything they can't do? :roll: Mind you it would have to be at least 1000W I'm guessing. Induction or brushed? If it's brushed then you could even buy one of those cheap electronic speed controllers to slow it down if it's too quick.

Hey I just remembered that I hired a spit once. Don't think it could turn 1700 between centres but reasonably sized. Pretty sure the shaft was less than 25mm. In fact it was square, maybe 3/4.

This will be a serious spit Brendan. Big enough for a forum spit roast! :D

Simon

[Stustoys]

The only spit I've had much to do with used 1" rod so I'm not much help there.
As you say the motor doesnt need much grunt, this one used a Holden wiper motor(which was pretty much the same for a lot of years). The idea being if it dropped dead there would be more than enough spare motors parked in the drive.:p
I only used it twice and didnt have to find a spare.

Stuart

[simonl]

To set your mind at ease Brendan, I found heaps on ebay like this one:
Heavy Duty Spit Roast Rotisserie Electric 80kg Motor Balance Weight | eBay (http://www.ebay.com.au/itm/Heavy-Duty-Spit-Roast-Rotisserie-Electric-80kg-motor-balance-weight-/230888920004?pt=AU_Outdoor_Living_BBQs_Cookware&hash=item35c20d53c4&_uhb=1)

Rated to 80Kg and is designed to take a 22mm shaft. So i'm guessing 40Kg on a 25mm shaft MAY just work! Couldn't find the power rating on the motor though. Bet it's less than 150W!

Simon

[19brendan81]

"The idea being if it dropped dead there would be more than enough spare motors parked in the drive.:p "

ha ha ha. I like that thinking.

Hey Simon - yeah I have seen those motors...might end up buying one. The spit we are building is turning out pretty nice and we are thinking of hiring it out after we are done with it. It might be worth having a nice motor on it for that purpose alone.

I still want to see if I can strap this washing machine motor to my gearbox...will be a good chance to use my lathe. I think that the motor spins at two speeds - 900 and 1400rpm....it would be perfect for the spit and should have plenty of grunt. Just need to figure out how to wire it up.

[j.ashburn]

"The idea being if it dropped dead there would be more than enough spare motors parked in the drive.:p "

ha ha ha. I like that thinking.

Hey Simon - yeah I have seen those motors...might end up buying one. The spit we are building is turning out pretty nice and we are thinking of hiring it out after we are done with it. It might be worth having a nice motor on it for that purpose alone.

I still want to see if I can strap this washing machine motor to my gearbox...will be a good chance to use my lathe. I think that the motor spins at two speeds - 900 and 1400rpm....it would be perfect for the spit and should have plenty of grunt. Just need to figure out how to wire it up.
30yrs ago army days we had a bush spit made from 44gal drums [205l] and wood fired driven with 12 volt wiper motor driving a small pulley to larger one on the spit shaft. It had an idler that adjusted tension as the pole could be set at 3 levels for final cooking the shaft ran in u shaped 1 1/2 strap the fat from the roast lubricated worked like that for years later passed from 1 group to another.Brains trust in the bush with heaps of time and resources as scrounged. all designed in a haze of Viscount smoke and V B haze.fond memories. John.

[19brendan81]

Thats cool John....I have used a mini spit powered by a wiper motor, it would spin 10 kilos of meat easily....but never thought it would be powerful enough for a 30kilo pig. Our spits have always been made from 44gal drums....they are just perfect for it.

[nearnexus]

Where you measure the flex in the shaft will make a big difference in runout.

With a spit I would expect the load to be either fairly uniform or only near the ends where the skewers hold the beast.

In that case deflection would be low. You wouldn't be applying all the force to the centre of the shaft.

Turning a large weight on ball races wouldn't take much power, especially as the motor will have to be geared down to keep the rotational speed to a crawl.

Rob

[19brendan81]

Yeah the load is spread right across the shaft...the heaviest bits (head and rump) are each located only 300mm from a bearing.

"Hi Brendan,
Hope you and the new family member are well."

Yep thanks Ueee, all going swimmingly.

[Grahame Collins]

Hi Brendan,

I have a bit of experience with these rigs. Mine is 20 mm solid shaft driven by an industrial motor and gearbox that runs at 1 rev per 70 seconds.I had the rear hind quarter of a steer on it with no problems.

The rig was designed for a shopping mill aircon unit butterfly arrangement that closes of aircon vents feed in the case of fire, hence the slow travel rate.

The wall thickness and span of your tube is a bit light as to be right on the border line of permanent deflection, just my opinion ,there's no calcs to support this notion..
Think about weight, span and the fact that its hot and potential . The gearbox can be geared down again if you want slower via a car starter motor pinion and ring gear. I have seen them run off the 12 volt starter motor,though they had a couple of spares as the motor heats up a bit- good for the bush though.

Could you consider reinforcing the tube by inserting a close fitting black iron (not gal) cheaper heavier wall pipe ? Also as mentioned ,you need something pointed on one end of your tube to pierce the pork.

Grahame

[19brendan81]

Hey Grahame,

My old spit was 20mm solid steel too...smaller span (maybe 1500?). It was fine like this. I have tonnes of solid mild steel at home that I could use, however as we plan on renting this one out we thought we better make the food contact part of this rig all stainless - hence the purchase of the 25mm tube.

I thought about filling the tube with concrete to stiffen it up - what do you reckon?

Re the point - no probs - have 1m of 25mm solid stainless bar at home to make the pointy end and the driven end.

My previous spit motor, bought from Adelaide Electrical in 2006, turns at 8pm for memory. I quite like it at this speed as I find if you slow them down too much you get heaps of fat drippings and big flare ups.

[19brendan81]

Well...I was wrong. There is way too much flex in this bar to use it as a spit. Would appreciate thoughts on....

1) filling with concrete to stiffen up. Will this work?

2) turning down a piece of 25mm mild steel to insert and stiffen up. I assume this will work, but it may be a hassle driving it in as an interference fit.

3) giving up, putting stainless tube into consolodated revenue and buying a new piece of solid stainless thats more suited to the job.


Brendan

[Oldneweng]

Well...I was wrong. There is way too much flex in this bar to use it as a spit. Would appreciate thoughts on....

1) filling with concrete to stiffen up. Will this work?

2) turning down a piece of 25mm mild steel to insert and stiffen up. I assume this will work, but it may be a hassle driving it in as an interference fit.

3) giving up, putting stainless tube into consolodated revenue and buying a new piece of solid stainless thats more suited to the job.


Brendan

I don't like to repeat myself but what about 3 lengths of 6mm SS rod evenly spaced and triangulated along the tube. A lot of spit bars you buy have 3 bits welded to them anyway. This will make it more difficult to arrange mounting tho. I set up a small spit on my BBQ which had the same problem after I changed the design. I had to put the open end into its support, push it thru a bit then put the other end into its support and push back the other way so the shaft extended outside for drive mount.

Dean

[simonl]

Hi Brendan,

I'm not sure if concrete would work. It spawls at high temps (although not sure if the spit would be hot enough to do this) and it may not offer much support once it cracks. You may end up with the weight of the concrete but not offering much extra in strength. WRT to the steel insert, it may not need to be an interference fit. Just a snug fit may do because it will still add strength once the SS flexes and takes up any difference in size. It would not even need to be round, square may also work and because it's only contacting at four points so still offering strength but sliding it in would be easier. The other option with round insert, drill holes in the SS at regular, measured locations and then through weld once the insert is in place (with SS electrodes) then neatly grind the weld back so you cant see it.

Just some thoughts

Cheers,

Simon

[Grahame Collins]

I thought about filling the tube with concrete to stiffen it up - what do you reckon?


Concrete has aggregate which makes it strong.You would never get the correctly evenly mixed aggregate right down the tube.I have no idea of what effect the 3 or 4 hrs of heat would have on the curing /strength. I does not seem a good idea to me.

Grahame

[GTVi]

Here is a picture of mine, I'm the King when it comes to Spit cooking...

https://lh5.googleusercontent.com/-ugaw25mgU8U/T4qUydfP8PI/AAAAAAAAACI/D-qJ4f2gKik/w1143-h857-no/IMG_6066.JPG

1. It uses a Washing Machine motor. This motor is 40 years old and still running. They don't make washing machine motors like this anymore.
2. Runs via a gear system, and also twin wheels to change speed on the fly
3. Solid spit stainless steel bar, I have done lamb (easy) and pig (heavy) no problems, no flex.

The trick is to ensure there is no slippage...i.e. the weight is concentrated at the hips and shoulders, not the middle. I support it with 3 U-bolts and wire.
Need to make sure it very very secure/tight. otherwise the body will twist (due to the torque contributed by weight) as its cooked.

As you can see with mine I have a cylinder (old hot water system split lengthwise) for the heat to be concentrated onto the lamb.
The picture above shows the first few minutes of putting it on. Soon after it changes color it is lowered, variable height easy to do while its cooking.

https://lh3.googleusercontent.com/-7d8bDVQONcs/T4qUzFLESpI/AAAAAAAAACM/cUaUFvsWshY/w1143-h857-no/IMG_6074.JPG
https://lh5.googleusercontent.com/-pZ6CT0fgTlM/T4qU127BsII/AAAAAAAAACY/dVB9COFci08/w1143-h857-no/IMG_6075.JPG
Let me know if you have questions.

[19brendan81]

Yeah I have gone off concrete. IM sure it was a tip I read somewhere but I no longer can see it working. I dare say it would crack up quicky and be useless.

Dean whilst triangulating may work it would make fabricating the tie off points (which need to slide up and down the bar) more difficult. If I am going to buy 6M of 6mm solid bar for triangulation I would probably prefer to buy 2m of solid bar of the correct diameter.

If I can get some solid mild steel cheaply I think i'll try insertit up the middle of the stainless....if I cant I'll just bite the bullet and buy a proper piece.

GTI - nice looking beast. I have done a few spits before and agree securing the properly is the absolute key....i'll never forget the first one we did (a lamb) which after about 1 hours cooking was thrashing around so badly we had to pull it off and try tie it down properly....no fun when the thing is burning hot.