270 Offy

[H Pearce]

Hi,

Are you still using the original flat cover on the pump? The clearance between the impeller and cover is critical, it needs to as close as possible any gap even a small one will cause a large loss in efficiency. This small clearance needs to follow the blades all the way down to the outside tip. Their needs to be some space for the water to exit the impeller, ideally a volute housing for proper flow. Best common example would be a turbocharger compressor, you can clearly see the shape/flow even from the outside. Different medium but same idea.
Regarding a gear pump, the main potential issue I can see is the lack of lubricant for the gears. Guessing the engine won’t be run for long so shouldn’t be a problem? Keep in mind as it is a positive displacement pump it can make allot of pressure, not sure if the water jacket would like that.
With a bit of tweaking the centrifugal pump should work fine...

 

[bluejets]

There is insufficient "water cut" section in your design.
Made the same comment to another vane water pump the other day.

https://www.homemodelenginemachinist.com/threads/demon-v8-multiplied-1-5.31580/page-6#post-332292

post #102

 

[gbritnell]

Terry,
While I agree that there is a certain amount of thermosyphoning of the water and therefore while running an engine the radiator does tend to heat up (model T cooling system) the introduction of a pump should assist in the cooling effort. Not having a CNC machine I machine my impellers with straight vanes but offset from the centerline to give the desired flow across the vanes. My pump body has the outlet tangential to the inside bore of the pump. I have made the impeller shaft bore two different ways, one on center with the housing bore and one with an offset center to form what has been referred to as a cutwater. I have never tested the flow rate of these pumps but even after just a few seconds of running an engine I can feel heat circulating through the radiator.
Your impeller design should work great but I agree with the other posters that you need to make your discharge port tangential to the pump bore.
Attached is the drawing for the pump on my 4 cylinder engine. This design has somewhat of a cutwater that is simpler to machine without CNC. The pump draws water from the block then pumps it to the radiator.
gbritnell

 

[mayhugh1]

I spent several days designing a gear pump and then, just before starting its machining, I began having second thoughts. The Offy's coolant system is a pump located in the bottom of a closed loop that's completely filled with coolant. Since the water columns at the pump's inlet and outlet are at the same height, it should be capable of circulating the coolant with minimum pressure across it.

Wall friction within the tiny head passages in the upper part of the engine could conceivably create a flow bottleneck and increase the pressure requirement of the pump, but this can be calculated using Poiseuille's law:

Q = (pi * r^4 * P) / (8 * n * L) ,

Where Q is the flow rate,
P is the pressure drop,
r is the radius of the restriction,
L is the length of the restriction, and
n is the fluid's viscosity.

Plugging in values for an 1/8" diameter restriction that's 6" long (typical of one of the Offy's head passages) and using the viscosity of water shows that only .0023 psi is required for each cubic inch per minute flow. Full-size automobile engines typically turn their entire coolant volume over one to two times per minute. For the same turnover rate, the Offy's 3 to 6 cubic inches per minute should require no more than .014 psi from the pump. (For a sanity check on the math, visualize blowing through a short soda straw.)

It doesn't seem likely that removing waste heat from the Offy's head can be significantly improved with a gear pump capable of producing tens of psi pressure. It's more likely that the pump would have to be severely throttled back to prevent coolant leaks from an already questionable head gasket.

Setting aside my first attempt, a well-designed centrifugal pump should have little problem circulating coolant through the Offy. I carefully considered the comments received on my first pump before taking another stab at it. I improved the tangential exit and water cut and added some semblance of a real volute. Since there was a definite advantage to retaining the same impeller diameter, space for the volute was created by increasing the diameter of the pump body and then notching it for clearance around the starter shaft. Although still not ideal, the volute's geometry was considerably improved. The height of the impeller was also increased by 30%. I don't normally like making so many simultaneous changes while working my way up a learning curve, but it's getting time to move past this part of the project.

A few material changes were also made to improve the pump's long term corrosion resistance. It turns out that aluminum and stainless weren't the best metals to put into wet contact. Both the pump body and cover are now 7075 aluminum with the impeller was machined from Delrin. An integral Delrin sleeve also replaced the front ball bearing, and the impeller can now limit its own thrust with minimum wear to the cover. The coolant will eventually become a 50/50 mix of anti-freeze and water which will provide some lubrication and corrosion resistance.

The number of impeller blades was reduced from seven to six and their thickness increased some 30%. The impeller has a 1/4" diameter pressed-in metal shaft with a rear end that remains supported in a ball bearing similar to the original pump. Silicone grease packing and an o-ring in front of this bearing makes up,the pump's rear seal.

Since I previously saw a performance improvement with fish-mouthed impeller blades, I machined two impellers for this pump. One has full height blades, and the other one is fish-mouthed. Research showed the backward curved blades I'm using (or else offset straight blades) are the best performers for non-compressible fluids. Without CNC capability the extra machining effort required for curved blades probably isn't worthwhile.

Testing...
The new pump performed much better compared with my first attempt. It visibly circulated water through the simulated loop at effective crankshaft speeds as low as 500 rpm. In this pump, the full height impeller performed 30% better than the fish-mouthed impeller. At an effective 5000 crankshaft rpm, the pump was capable of producing a 12" water column (0.43 psi) with the full-height impeller and a 9" column (0.32 psi) with the fish-mouthed impeller. While producing a 3" water column, the full-height impeller flowed 54 cubic inches/min at an equivalent 5000 crankshaft rpm. At 500 crankshaft rpm it produced .040" psi while pumping water at 5 cubic inches/min. Even at such an optimistically low idle speed, this new pump should have no problem pumping coolant through the Offy. - Terry

 

[dsage]

Looks good. As usual. Thanks for sharing all the details.

 

[propclock]

Fantastic! I have been following and was very interested in this result.
all the model pumps I have seen/built have close tolerances from the impeller to the case .
Thanks for all your R&D. a true fan of yours.

 

[awake]

I love your detailed testing and reporting of results - and glad you got it working!

 

[Charles Lamont]

I like the new pump and am pleased you are now getting a satisfactory output. Out of curiosity I took your 54cu in/min flow and 3" head and calculated the power output (Q x P), but I am having difficulty believing my own figures. I make the flow rate 0.015 litre/sec and the pressure 0.00736 bar, from which I get a power output of 0.01 watt !?

 

[mayhugh1]

I like the new pump and am pleased you are now getting a satisfactory output. Out of curiosity I took your 54cu in/min flow and 3" head and calculated the power output (Q x P), but I am having difficulty believing my own figures. I make the flow rate 0.015 litre/sec and the pressure 0.00736 bar, from which I get a power output of 0.01 watt !?

I got the same results. I didn't know what to expect for the pump power. I guess the engine shouldn't have any problem handling it. Interestingly, if the water is replaced by 40W oil, the power required for the same flow rate theoretically jumps up to 2 Watts. - Terry

 

[Charles Lamont]

I have thought about it a bit more and, surprisingly, it seems reasonable, if you see what I mean. It is worth remembering that Earth's gravity exerts on an apple a force of about one Newton. To lift an apple a metre in a second requires a Watt. So lifting a tablespoon of water three inches in a second is going to be quite a lot less. I think it has to do with human perception of different forms of energy: a watt of heat is nothing, a watt of light miserable, a watt of sound almost adequate, and a watt of mechanical work quite appreciable. So a small engine will pump what we perceive as quite a lot of water but will light only a few tiny bulbs. BTW, your pump will also be horribly inefficient, and has friction in the bearing and seal, so it will need more than 0.01W to drive it.

 

[mayhugh1]

Calorie consumption is another quantity whose perception borders on the unfair. In the gym last night I sweated on a stair stepper for half an hour. When done, it indicated I had climbed the equivalent of a 90 story building and in the process consumed only 175 calories - barely what was in your apple. - Terry

 

[awake]

Clearly we need to petition our governments to pass legislation redefining the value of a calorie!

 

[mayhugh1]

The starter yoke is the carrier for starter shaft's forward bearing, and it's the engine's attachment point to the front motor mount. Being a prominent and distinguishing feature of the engine, I didn't want to alter its appearance. Although relatively straightforward to cast, the yoke would be a difficult part to machine from billet even with access to CNC. I purchased the pdf version of the files just before beginning this build, but I in fact bought a printed copy of the manual nearly twenty years ago just to learn how Ron had made this particular part.

Remarkably, it was to be machined from an aluminum bar with a 3/4" starting thickness before being formed around a steel mandrel using heat and a mallet. Ron didn't specify the alloy he used, but after researching the (non)availability of suitably thick 3000 or 5000 series bendable bar stock, I concluded he probably just used 6061. A full-size template is provided in the drawing package, but after cutting out a cardboard replica to trial fit to my partially assembled front end, I discovered a nasty interference with my redesigned water pump.

The outlet on Ron's pump exits below the starter yoke. The relocated outlet on my pump improved coolant flow but was now blocked by the side of the yoke. In order to clear the interference, the pump had to be rotated at least 20 degrees clockwise to get the outlet under the yoke. Each degree of pump rotation added a degree to a second angle that was going to have to be added to the pump's already tightly bent outlet tube. The clearance slots added to the pump for the starter shaft required the machining of a new pump body and cover. Since the pump's internals weren't modified, though, its previously measured performance should remain intact.

The starter shaft will eventually be supported in a bronze bearing pressed into the yoke, and so the yoke needs to be accurately formed and positioned on the engine so the starter shaft winds up collinear with the crankshaft.

Work on the yoke began with experiments to form 180 degree bends in 1/4" thick 6061 scrap bar stock. I constructed the mandrel suggested in the plans which was designed undersize to accommodate spring back. My initial tests showed negligible spring-back, and so I made a new full-size mandrel. I also added a shoulder bolt to securely anchor the workpiece in a guide slot machined into the mandrel. It's very important that the final part be symmetrically formed around the bore for the starter bearing. The addition of the mounting bolt adds consistency to the fixture, and it provides for inevitable alignment tweaks after the yoke cools.

My first attempt at a 180 degree bend was performed on a 1/4" piece of scrap that had been annealed in an oven at 775F for two hours. It bent smoothly without opening up its grain structure, but the heavy hammering required would most likely damage the delicate features on the actual part. My next attempt was to apply heat to the workpiece using a torch only along the bend lines and at a significantly higher temperature so the part could be formed using much lighter hammer blows. Some practice was required to avoid heat damage to the part which my large acetylene rosebud tip tended to do. My less brutal mapp torch just couldn't deliver enough heat. Forming was done using a two pound metal hammer buffered by a piece of 1" x 3" oak.

After gaining experience with scrap bar stock, the actual pre-machined yoke was easily formed on the very first try. My experiments didn't come up with an estimate for a stretching allowance, and so the arms of the yoke were machined long and trimmed later to fit. My wife assisted with the torch while I was busy with the hammer and oak plank. A machinist square insured the initial fit was very close, and only minor tweaking was needed to get the yoke to slide onto the front of the engine in perfect alignment with the crankshaft.

The installation of the bearing and the mounting of the yoke to the engine will be completed after the starter shaft is machined. - Terry

 

[dnalot]

Nice job bending aluminium. I am a aircraft & power-plant mechanic. I have had many occasion to repair or replace damaged parts that were formed. We would send the material out to be annealed to a soft state before we would form it and then send it out to be heat treated for hardness. And then we would reform the part again because the heat treating process would warp the part a little. Again , very nicely done.

Mark T

 

[mayhugh1]

Before beginning work on the starter shaft, I tied up a few loose ends on the engine's lower half. Although I haven't yet given any thought as to how the engine and its running accessories will be mounted, I machined the front and rear motor mounts. They'll come in handy later while working on the top end. I also machined the drive tenon on the end of the water pump's impeller shaft so I could finally assemble the new pump.

Returning to the starter shaft, I drilled clearance holes through the sides of the yoke for its mounting screws and then transferred their locations to the crankcase halves for drilling and tapping. A test rod used during this step insured alignment among the yoke's starter shaft bore and the main bearings in the crankcase. With the yoke in place and its bronze bearing installed, the starter shaft's dimensions could be verified before it was machined. A last minute decision was to not permanently install the bronze bearing in the yoke. Instead, it will be held in place with a setscrew hidden by the front motor mount. This will allow the bearing to be replaced, if ever necessary, without having to make a new yoke.

The starter shaft was machined from 1144. During cranking, a one-way bearing pressed into its rear end will grip the nose of the crankshaft which will also be machined from Stressproof. During my Knucklehead build I learned something that Ron already knew: 1144 isn't hard enough to withstand the rigors of a sprag clutch. His solution was to Loctite a hardened ring onto the nose of the crankshaft. Since I was already working in the area, I machined and hardened a couple of these rings for use later on. The original drawings show a .060" deep clearance flat that must be ground on this ring to allow the nose of the crankshaft to slip past the 60 tooth oil pump drive gear on its way into the front cover ball bearing during final assembly. The plans also call for adding a replacement piece back into the ring after assembly. This bit of voodoo initially raised concern, but fortunately the split crankcase allows an assembly sequence that totally avoids the need for a notch.

After assembly, the starter shaft is held captive inside the yoke between a thrust surface on its bronze bearing and the main ball bearing in the front cover. A 3/8" hex machined on its outside end will be gripped during cranking by an adapter in a battery-powered drill.

Care was required during installation of the one-way bearing in the end of the starter shaft. Not only must it be installed in the proper direction to crank the engine clockwise (when viewed from the front of the engine), but the actual bore diameter is very important. The .750" diameter bearing is designed to be pressed into a .750" diameter bore regardless of what one is likely to measure as the o.d. of its thin drawn metal shell. The bearing's outer race depends upon it being installed in the proper size bore. A short length of rod stock inside the bearing adds some extra support during the pressing operation which, of course, must be accurately started to avoid damaging the bearing.

I'm looking forward to the next mini-project: the magneto which, like the Merlin's, will be a distributor in disguise. - Terry

 

[stragenmitsuko]

a true work of art ...

 

[RonC9876]

Terry: That Voodoo notch became necessary when the original smaller starter clutch was unable to handle the compression load for any length of time. I kept going larger and the interference with the gear became a problem. Cutting the notch and adding the piece back in works quite well and has never caused a problem. I fretted over that modification but it worked out well!

 

[PhilWroundrockTX]

Before beginning work on the starter shaft, I tied up a few loose ends on the engine's lower half. Although I haven't yet given any thought as to how the engine and its running accessories will be mounted, I machined the front and rear motor mounts. They'll come in handy later while working on the top end. I also machined the drive tenon on the end of the water pump's impeller shaft so I could finally assemble the new pump.

Returning to the starter shaft, I drilled clearance holes through the sides of the yoke for its mounting screws and then transferred their locations to the crankcase halves for drilling and tapping. A test rod used during this step insured alignment among the yoke's starter shaft bore and the main bearings in the crankcase. With the yoke in place and its bronze bearing installed, the starter shaft's dimensions could be verified before it was machined. A last minute decision was to not permanently install the bronze bearing in the yoke. Instead, it will be held in place with a setscrew hidden by the front motor mount. This will allow the bearing to be replaced, if ever necessary, without having to make a new yoke.

The starter shaft was machined from 1144. During cranking, a one-way bearing pressed into its rear end will grip the nose of the crankshaft which will also be machined from Stressproof. During my Knucklehead build I learned something that Ron already knew: 1144 isn't hard enough to withstand the rigors of a sprag clutch. His solution was to Loctite a hardened ring onto the nose of the crankshaft. Since I was already working in the area, I machined and hardened a couple of these rings for use later on. The original drawings show a .060" deep clearance flat that must be ground this ring to allow the nose of the crankshaft to slip past the 60 tooth oil pump drive gear on its way into the front cover ball bearing during final assembly. The plans also call for adding a replacement piece back into the ring after assembly. This bit of voodoo initially raised concern, but fortunately the split crankcase allows an assembly sequence that totally avoids the need for a notch.

After assembly, the starter shaft is held captive inside the yoke between a thrust surface on its bronze bearing and the main ball bearing in the front cover. A 3/8" hex machined on its outside end will be gripped during cranking by an adapter in a battery-powered drill.

Care was required during installation of the one-way bearing in the end of the starter shaft. Not only must it be installed in the proper direction to crank the engine clockwise (when viewed from the front of the engine), but the actual bore diameter is very important. The .750" diameter bearing is designed to be pressed into a .750" diameter bore regardless of what one is likely to measure as the o.d. of its thin drawn metal shell. The bearing's outer race depends upon it being installed in the proper size bore. A short length of rod stock inside the bearing adds some extra support during the pressing operation which, of course, must be accurately started to avoid damaging the bearing.

I'm looking forward to the next mini-project: the magneto which, like the Merlin's, will be a distributor in disguise. - Terry


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[PhilWroundrockTX]

Hey Terry, I'm always in awe of the art/perfection/creativeness/etc of your work and really enjoy the detailed documentation of your processes along with fine photos of projects both in progress and finished pieces. Know I'll never get to your level of work but am inspired to improve both my projects and start keeping notes to help in future similar efforts. Thanks and can't wait to see and at some point hear the Offy!

 

[Tim1974]

Same here I love the thread allways good reading and very nice work