The Steamboating Forum

Howdy All,

I am considering a flexible coupling for my set up and have what is probably a naive question. I have a 6 hp engine and calculating the torque through various rpms. The range varies from 52 foot lbs at 600 rpm to 315 foot lbs at 100 rpm. I am trying to decide how robust my coupling needs to be. The hardier they are, the more they cost and so I am trying to avoid my typical urge to just buy the most expensive one. College bills are cutting into my steamboat fund.....

315 ft lbs works out to 3780 inch lbs. My highest option before jumping up significantly in cost is 2200 in lbs, or 183ft lbs. which would translate to the torque at 175 rpm.

Relating to Winnie the Pooh as a "Bear of very Little Brain"..............I am having trouble believing that I will put that much strain on the coupling spinning a prop in water as opposed trying to turn a wheel against the ground.

Comments..........

jon

The calculation of 6HP, 100 RPM, and 315 pound-feet of torque is correct, however at this low an RPM your engine could not produce anything near 6HP.

Your 3 x 4 Double acting single cylinder Tiny Power engine would produce about 5.4 HP at 350 RPM with 135 PSIG steam, and about 68% cutoff. That is 81 pound-feet of torque on average. The maximum torque with 150 PSI on the piston and 4 inch stroke is 177 pound-feet. This illustrates the maximum torque (stall torque) compared to the average torque when running properly.

The flywheel absorbs and gives back energy on every engine revolution, and smoothes out the variable engine turning torque to a value close to the average output torque, 81 pound-feet. For design purposes with a reasonable margin you need a coupling that is rated for about 100 pound-feet, or 1200 pound-inches. Of course if the propeller hits a log and is almost instantly stopped, the torque will be considerably higher for an instant of time.

If the low price coupling is rated to allow 2200 pound-inches, then that would be the one to buy. Looking at various suppliers, the flex coupling should cost something less than $40USD. What brand and type of coupling are you considering?

Thanks Fred !

That makes more sense. I was holding the horsepower constant. I am looking for a coupling that will allow me to mate a 1" shaft on the engine to a 1 1/4 " prop shaft.

I have been considering McMaster Carr Item 6241K23, a flexible tire coupling. The total cost would be around $350. If I go for a lower rating unit it would be around $200 Item 6241K12.

Can you point me toward a source that is less expensive ? I am hoping for a little "forgiveness" from the flex coupling to keep vibration down if I am not aligned perfectly, which I wont be.

jon

JonRiley56 wrote:Thanks Fred !

That makes more sense. I was holding the horsepower constant. I am looking for a coupling that will allow me to mate a 1" shaft on the engine to a 1 1/4 " prop shaft.

I have been considering McMaster Carr Item 6241K23, a flexible tire coupling. The total cost would be around $350. If I go for a lower rating unit it would be around $200 Item 6241K12.

Can you point me toward a source that is less expensive ? I am hoping for a little "forgiveness" from the flex coupling to keep vibration down if I am not aligned perfectly, which I wont be.

jon

If by 'flexible coupling' you are refering to a C/V joint, or even a U/J then you are after the wrong item to to cancel out vibration due to misalignment, and will in fact contribute to the problem as the shaft centreline will oscillate. C/V's and U/J's are designed to provide a break in two shafts that are aligned on centerlines that are at a shallow angle and low RPM and torque (in the case of U/J's) or constantly changing angle (within parameters) and higher torque levels/RPM (in the case of C/V joints.)

The question you need to ask yourself is why you are willing to spend hundreds of dollars on a joint that won't solve a problem that itself can be solved by a few hours of careful thought, an accurate rule, a bit of plywood and sharp block plane?

Greg

As Fred says, the formula for power and torque will tell you a lot. Our steam plants are different in that they can provide maximum torque at zero speed but that certainly doesn't mean that the torque increases without limit as the speed drops. That is true in theory of some electric motors but the smoke always escapes first.

I agree with Greg. You really shouldn't need a flex coupling. We've been running our marine power plants both I.C. and steam for years without anything but decent alignment.

If you design your engine mounts so that they are easily adjustable you will make your life sooooo much easier. The dolt that built my scow set the engine on stacks of washers and lag bolts. What a monster to get it right! High on my list of big improvements is to change this to a system where I can move the engine around with a wrench. Why the hell he couldn't have just bought four big fat hanger bolts and some nuts and done it right is beyond my comprehension.

Harumph!

Mike

A flexible tire coupling is a fairly expensive option, I was thinking of the conventional LOVEJOY couplungs, in the $40 range more or less. Check Graingers, or better yet, Surplus Center. These have two steel pieces, you can order each for the proper shaft diameter. An elastomeric "Star" shaped piece fits into (between) the two halfes, and allows some level of mis-alignment without hardship, perfectly adequate on many steamboats. What is taking the propeller thrust load, another bearing in the boat? I do not remember from last September.

You should align your propeller shaft to the engine shaft as well as possible, but with flexability of a hull a coupling should be used on your boat. (Do you remember how badly it vibrated when first started, and how very much better the vibrations got after an "eyeball" alignment at Lees Mills last September?)

Those Lovejoy couplings are good. I have one between my engine and Hypro feed water pump. Bart had one that squeaked. Probably just a mouse.

It allows me to remove my pump easily. I hadn't thought of one on the prop shaft. On Folly there is a separate (and redundant) bearing taking the thrust. Thus the shaft is over determined; cutlass bearing, thrust bearing and engine ball bearing mains. I could add one of those couplings and let things self align.

The L150 series will surely take the torque and maybe even the L100 would do. There are several on EBay just now for fairly cheap.

Thanks for the idea Fred.

Mike

Thanks Fred !

I dont have a thrust bearing in my drivetrain. The propshaft goes straight to the stuffing box.
I will take a look at the lovejoy types. I actually just got a surplus center catalog this week.
God works in mysterious ways.....smile

Hi Jon,

As stated above, the best thing to do is just align the engine and use a rigid coupling.

To adjust the engine, use a tapered hardwood wedge under each corner of the engine mount - old tried and true method for mounting inboards. Of course the production boats they got all proper and used cast iron wedges. What angle to cut the wedge? The same angle as the propshaft is to the engine bed. To find it use an angle gauge or protractor-dividing head etc. One leg on the prop shaft - the other eyeball it to the engine bed with a stick. Wedges will provide an infinite amount of adjustment between their ends. How thick? The best thing to do is shore the engine up close to where it will eventually rest and take measurements.

To align the couplers: If you have a shaft log and flexible mounted stuffing box, make sure the shaft is not laying against the bottom of the tube, prop it up if near center if needed. With the coupler halves mounted to their respective shafts, move them close to one another and start checking the gap between them with a feeler gauge. Tap the wedges back and forth until the halves are aligned parallel with one another. Then put the bolts in the coupler. Then just drill a hole through the wedges for the engine mounting bolts. Tightening is going to compress the wedges a little bit. I try to leave the engine just a little high, about .020". As you're tightening, roll the engine over and make sure it is not binding.

Rigid couplers go for all sorts of prices, should be able to find one in the thirty dollar range.

A note on mounting the halves. They typically come with set screws, best thing to do is drill straight through where the set screws go - all the way to the other side and put a bolt in it with a locknut.

-Ron

Looks like the propeller thrust is to be taken by the crankshaft bearings of the engine, in that case you need a coupling that can take both forward (compression) and reverse (tension) thrust, plus the turning torque of the drive system. The Lovejoy coupling is good for torque and compression thrust, but not good for reverse (tension) thrust.

As mentioned above, the solid coupling you already have is good for your installation, as long as the alignment is done with reasonable precision.

I would recommend you buy the Surplus Center "Double Roller Chain Coupling", Item I-3560, for $29.99. It will be good for all your loads, allows easy disconnection of the engine from the prop shaft without disturbing alignment, and gives some measure of alignment compensation. The price is hard to beat, and they offer the shaft sizes you need.

It should be installed and aligned as stated above. The two set screws that do not hold onto the key should be fitted into shallow (about 1/16 to 1/8 inch deep) drilled recess on the shafts, so that the thrust forces will not allow the prop shaft to be displaced when thrust loads are applied.