Suzuki transmissions

Fox, technically it's a 'drive fit' and it's suprising how strong it is. The gear gets heated to expand it and the shaft is cooled to shrink it. The cooling is usually done with liquid nitrogen which is -321 °F. The rate of expansion for steel is approx. 6 millionths of an inch per inch per degree F (Aluminum is approx. 13 millionths). If the shaft is 1" at 70°F, it will shrink 0.000006" x 391 degrees x 1", or about 0.002346". Add to that another .001" expansion achieved by heating the gear and you end up with close to .0035" additional clearance between the parts. This allows the shaft OD to be .003" larger than the ID of the gear when the temperatures equalize, yet are easily assembled without damaging or distorting the parts. A .002" interference fit is in the range for a FN2 drive fit which is sufficient for the torque loads expected provided there isn't high lateral loading present.

The reason they used it is simple - it's cheap! The only machining required is the shaft turned to size and the gear bored to size. Splines and even simple keys are much more expensive to machine because they require additional operations. The biggest part of an engineer's job isn't making something work, it's figuring out how to make it as cheaply as possible while maintaining the quality standards set by management.

Lets just say that the shafts on ships are usually attached this way. Splines and keyways generate stress raisers that can lead to cracks. This method holds up to millions of foot lbs of torque.

whenfoxforks-ruled said:

Do other applications "spin seize" also?

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I'm not sure what you mean by "spin seize". Can you elaborate?

Fox, it sounds as though there may have been other factors that caused the failure of the press fit. The added oil porting on the new gear indicates they were experiencing lubrication issues in this area. OTOH, for the drive fit to work properly, the ID & OD dimensions are critical. Too much clearance it will slip, too little the gear can fracture. It's certainly possible they may have had QC issues which led to poor fits and eventual failure due to slippage and the reason they abandoned the process. A keyway would only be used in lighter press fits that are meant to be seperable without special techniques. As GMC pointed out, keyways create a weak point that can lead to fracture failures. Drive fits (aka shrink fits) are considered permanent and not easily seperated without destruction of the outside element. When properly designed and machined, they are capable of transmitting surprising amounts of torque and FN3 fits can also tolerate lateral loading.

Repair of the older RM is going to be tricky. I don't see the metal being that big of an issue as the Japanese tend to use very high quality steels in their motors and transmissions, and junk steel in just about everything else. Most of my experience has been late 70's on so it may have been different in the mid 70's. I think the bigger issues are going to be preventing the welding from warping the shaft, and then accurately grinding the shaft afterwards.

Bunya said:

It's certainly possible they may have had QC issues which led to poor fits and eventual failure.

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Doesn't this happen to ALL suzukis :rotfl: