This coupling was originally designed to fit between the engine drive flange and the propeller shaft on an inboard boat engine. It utilizes a splined coupler to transmit loads, the steel components are factory made and assembled to provide high torque loads along angular misaligned shafts. They are easy to assemble and disassemble in the field; and they are also well suited for such applications as fans, blowers, pumps, compressors, air conditioning and refrigeration equipment, and other rotating equipment.

Why do you need it ?

The rotating drive on an engine is not always aligned to the equipment being driven. Most engines are mounted on elastomer engine mounts to isolate vibration, while the equipment being driven is often hard mounted. For instance, in a boat, a propeller shaft is attached to the transmission using a flange mounting, and then extends through the boat hull using a water tight seal and cutlass bearing. Since one end of the propeller is fixed by the hole through the boat hull, it is important to ensure that the engine and propeller shaft are closely aligned. The problem is: it is not always easy to align the engine and propeller, and once aligned, over time the elastomer engine mounts tend to sag causing additional misalignment to occur.

How does it work ?

The Spline Shaft Coupler contains an internal female spline shaft with a small space at each end. A short shaft with a male spline at one end is inserted through the coupler flange; with a small rubber pad placed on each end. The splines engage, transmitting the engine torque from the coupler flange to the spline shaft. The splines are made with .01 inch of clearance which absorbs a couple degrees of misalignment between the flange coupling and spline shaft. The rubber pads at each end facilitate the misalignment and keep the parts pressed smoothly together, avoiding any tapping or rattling.

• Prepacked - No lubrication required.
• Splined shaft design optimized for torque capacity and life.
• Sizes and types available to replace ridged joint and elastomer flex couplings.
• Wide range of flanged coupling sizes and types available.
• 2 Degree Misalign.
• 5 Year disc warranty.

Technical Data

Recommended torque settings for lubricated threads (light oil) 5/16 UNF - 21.5 Nm(16 lbs ft) 3/8 UNF - 33.5 Nm(25 lbs ft) M10 - 40 Nm(30 lbs ft) 7/16 UNF - 47 Nm(35 lbs ft) 1/2 UNF - 74 Nm(55 lbs ft) 5/8 UNF - 154 Nm(115 lbs ft)

My own experience installing the Spline Flex Drive in my 35.5 ft Hughes Sloop

(Atomic Four V-Drive):

I unbolted the prop flange from the transmission flange which involved removing 3 each 5/16 bolts. I slid the propeller shaft back about 5 inches to give myself some additional space. I then placed a 1 inch band clamp around the propeller shaft as “extra protection”; after all, I did not want the propeller sliding out through the packing box, cutlass bearing and falling into the deep blue sea.

I removed the two set screws that hold the prop flange in place, and lightly tapped the flange until it slid off the end of the propeller shaft. I picked up the keyway that had fallen off the shaft, and put it away; incase I should need it again. I then place the Flex Spline coupling in place up against the transmission flange and made sure the Coupling Flange would fit, and all the holes lined up; everything looked “OK” so I decided to keep going. Next was the hardest part.

I wanted to maintain the same propeller location relative to the boat hull. That meant, I needed to shorten the propeller shaft by about 2.5 inches. I got a hand grinder and installed a thin abrasive blade, I also purchased a couple extra blades just incase I broke one (they cost about $2.00 each). I then spread a wet towel all around the work area to catch any hot sparks. I also ended up removing the side handle on the grinder (it threads off) to give myself better access. I then put on a set of safety glasses and proceeded to cut 2.5 inches off the end of the propeller shaft. It took less than 5 minutes to cut the shaft, the thin abrasive blades held up better than I expected, so I only needed one to do the job. I then took a small hand file and smoothed the end of the shaft – deburring the sharp edges. I cleaned up my work area, mounted the Spline coupling using the same 3 bolts I had removed earlier, slid the shaft-to-shaft coupler onto the end of the flex coupling. I took an extra minute to be sure the shaft-to-shaft coupler was centered. I then slid the propeller shaft forward into the shaft-to-shaft coupler; and tightened all the bolts.

I was anxious to do some testing so I cranked up the engine put into forward and then reverse several times, straining the dock lines as the propeller thrashed a lot of water around. I left the boat in gear and went below to have a look at the spinning shaft - all looked OK. After about 10 minutes, I stopped the engine and checked all the bolts and looked for any slipping, again - all looked OK, so feeling a little more confident, I removed the band clamp I had put around the propeller shaft. For the next two days, I took the boat out and pushed the engine pretty hard. I had a pleasant surprise, I used to have a prop shaft “wobble” that seems to have gone away, and the engine showed improved, smother performance; the prop shaft must have been more out of alignment than I thought. At the end of the second day I took the flex coupling apart and looked it over. I saw no signs of wear so I reassembled everything. As 5/14/2008 I’ve put 284 hours on the engine since the flex drive installation on 4/15/2005.

I designed this flex coupling myself, after unsuccessfully trying to buy one to fit my boat. I’m an Aerospace engineer by profession, so I had some design tools at my disposal. I did the design and detail drawings using the software routine SolidWorks, and then performed the structural analysis using COSMOS.