Model Engine Maker

Engines => Your Own Design => Topic started by: gbritnell on January 19, 2019, 11:31:03 PM

Title: Tiny Holt 3 cylinder revisited
Post by: gbritnell on January 19, 2019, 11:31:03 PM
 With winter hard upon us in this part of the world it's time to fix, repair, modify or build something. I just completed the drawings for the Galion road grader and will be starting on it after all the aforementioned tasks are complete.
 When I built the tiny Holt I wanted to replicate the larger engine only in 1/2 scale. As a lot of you know scaling down isn't just taking the drawings and multiplying by .5. There are things that worked in the larger size that just won't translate when you miniaturize some of the parts.
Most everything, screws, wall thicknesses, gears and such only took minimum modifications to create the smaller version. I wanted to use a water pump similar to what I had made for the full sized engine, which is driven by the appropriate length O-ring. When I made the pulleys 1/2 size I found out that the smaller size didn't provide enough surface area for the belt to grip. The fan works great but there is little or no resistance to spin it. The pump on the other hand is a gear type (2 spur gears running together in a close fitting chamber). There is the friction from the gears and also the close fit of the shaft to the bearings.
 The first modification that I did was to enlarge the driven pulley. This necessitated reworking the pump, or should I say remaking the pump and moving the water inlet farther out from center. This was only a marginal improvement. Sometimes it would turn but once a little oil or coolant would got on the belt it would just slip.
 I ran the engine at several shows through the summer and just took the belt off. It wouldn't thermo-syphon very well because the coolant wouldn't flow past the gears.
 The remedy would be to have a more solid connection between the crankshaft and the pump. The only answer was gears.
I set about designing a way to run a gear train down to the pump. Knowing that there isn't much resistance the gears only needed to be .062 wide. I decided on 48 DP, for which I have several home-made 'hob type' cutters. The gear on the pump is limited in diameter because it needs to clear the inlet hose and clamp and the gear on the crank has to be large enough to get 2 0-80 flat head screws between the crank bore and the root diameter of the gear.
I would use the existing 3 mounting holes for the timing gear case at the top and at the bottom (pump end) I would use the 2 mounting holes that hold the pump to the mounting leg. My goal was to keep the gears all the same size so I wouldn't have to keep changing the dividing head plates while cutting the gears but in laying the gear train out I found that the #4 gear center fell almost on top of one of the pump mounting holes. I enlarged this gear until I could clear the head on the 0-80 soc. hd. screw and use a 2-64 thread for the gear axle shaft. Now I have 3 different sizes, pump gear, crank gear and #4 gear. I also needed 5 gears in the train to keep the pump turning in the correct direction to pump water. The remaining space was filled with 2 smaller gears, but the same size. One less dividing plate change.
The first picture shows the front of the engine with the radiator, fan bracket and flywheel removed. 
Title: Re: Tiny Holt 3 cylinder revisited
Post by: gbritnell on January 19, 2019, 11:37:21 PM
The next picture show the gear case mounted to the engine. I milled a piece of aluminum to size then drilled and tapped the necessary holes. The counterbores were put in with the boring head and the central slot was milled with enough clearance for gears #2 and 3.
It took a little finessing to get the gear housing to fit right because the pump face and timing cover face calculate out to be .012 but a thou here or there makes for too tight or loose of a fit.
Title: Re: Tiny Holt 3 cylinder revisited
Post by: gbritnell on January 19, 2019, 11:45:56 PM
Once I had the gear case nicely fitted up I cut the sides away to make it a little more interesting to look at.
Next up were the gears. I cut blanks for each gear, steel, brass, steel, brass and steel. This was so I didn't have the same material running against itself.
Each gear required a different threaded arbor.
The blanks were mounted in the dividing head and indicated true. The required dividing plate was installed and the gears were cut.
As I mentioned earlier the gears are .062 wide and that's what they were cut to. In fitting them to their respective arbors some needed a little thinning so when the arbor was tightened they still spun freely.
The gear on the flywheel is held in place by 2, 0-80 flat head screws. The diameter of the head had to be reduced so they would clear the cankshaft.
Title: Re: Tiny Holt 3 cylinder revisited
Post by: gbritnell on January 19, 2019, 11:50:35 PM
The last 2 pictures show the flywheel installed then the fan bracket. You can't see much of the gear train except for the #4 gear and the one on the pump.
Now all I have to do is wait for a warmish day to go out in the garage to see if the water flows like it's supposed to. That will probably be around the end of March or first of April.
Title: Re: Tiny Holt 3 cylinder revisited
Post by: toolznthings on January 20, 2019, 01:14:30 AM
Hi George,

You must have been a watch maker in another life !  Amazing work as always !

Happy New Year, too.

Title: Re: Tiny Holt 3 cylinder revisited
Post by: Craig DeShong on January 21, 2019, 12:49:58 AM
Hi George, nice to see you cutting metal again.  Your work is exemplary as always.   :ThumbsUp:
Title: Re: Tiny Holt 3 cylinder revisited
Post by: gbritnell on January 21, 2019, 03:28:59 AM
Thanks Brian and Craig,
It's really hard to be away from the tools for any amount of time. I really enjoy the designing and drawing but the metal kind of gets in your blood.
Title: Re: Tiny Holt 3 cylinder revisited
Post by: fumopuc on January 21, 2019, 08:46:12 PM
Hi George, great to see you back in the shop.