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Member postings for John MC

Here is a list of all the postings John MC has made in our forums. Click on a thread name to jump to the thread.

Thread: A Question on Bench Blocks
26/12/2019 08:10:47

I'm guessing the answer to the OP's question is "poorly made".

I've had a Starrett bench block on my bench(es) for longer than I can remember, a useful thing to have. if you don't then its a couple of productive hours to make one.

Not sure what makes the block precision, other than the parallelism of the top and bottom surfaces, thats been useful on the mill a good few times.

As for using it as a drill and tapping guide, the Sparber article, surely using it inverted would be better?

John

Thread: Merry Christmas Everybody!
25/12/2019 09:05:29

Merry Christmas!!!!

Just enjoying the peace of this Christmas morning before the family invade.......

Thread: Nalon Viper
24/12/2019 09:44:31

Old School, as you say, design of the bearing housing doesn't seem to have changed much in a long time. I cannot help but think that some of that is due to a lack of understanding of the principles of rolling element mounting.

I would like to know why wide clearance (hybrid?) bearings are being used?

Please don't call it my design, I claim no originality for the design, its good bearing mounting practice and I've seen it used on several engines.

My current project is using a bearing arrangement copied from an engine that was designed and built some 40+ years ago. If memory serves the aim was to get 1bhp from 5cc's, (may have been 10cc's), I think the aim was achieved. I'm no longer interested in sharing this design. I may incorporate it in to a Nalon Viper in the future as there are some aspects of the engine that seem to be good.

The pleasure I derive from occasionally dipping in to the world of small I/C engines comes from the design, manufacture and getting it to run aspects. My aero modeler friends mount them in aircraft to see if they make the power to fly. My present project is going to be tried in a tether car. The engine is not a high performance job, never the less it will be satisfying to see it do a few laps.

John

24/12/2019 08:00:39
Posted by Roy Vaughn on 23/12/2019 11:02:23:

John, you seem to be suggesting that the rear outer race should be free to move in the housing. This is exactly the condition that must be avoided at all costs with this type of motor. As Rob M says in his article, the hammering the rear bearing gets will eventually loosen the outer race if it not securely fitted, friction will build up and performance will be lost. High performance motors which use standard ball races often use a nut to secure the inner to the shaft. The philosophy nowadays is to clamp everything up so it can't move. The expense of shimming is the price to be paid.

So, the recommendation is to impose a preload on the bearing to try and stop them moving in the case? Looking at the lengths these guys go to extract the last few watts of power from these engines it seems to me to be counter productive.

Frankly, it looks like some engine designers have gone down a non-productive alley with design. While some of what they do is impressive in terms of power to weight ratio, the designs are not so good. Time for a rethink perhaps.

23/12/2019 08:15:48

I've just read through the service sheet for the MB engine. Like the idea of incorporating one of the inner races in to the crankshaft, a simple solution to the vexing problem of clamping one of the inner races to the shaft.

Why, I wonder, is there any need to use shims to control end float of the shaft? In the area I have circled in the (copied) sketch, if there is a small amount clearance between the case and the bearing the need to him would be eliminated. As a bonus the tolerances on the various linear dimensions associated with the components could be eased somewhat.

As Emgee has pointed out the heat required to disassemble the engine suggest the bearing won't move in use. What will happen is that the bearing will be able to find its own position during running, therefore not imposing any axial load on its self. The other bearing is doing all the axial location, just as it should be.

mb engine3.jpg

22/12/2019 08:13:30

A few points. Wide clearance bearings are readily available in most sizes at only a very small price premium. Its not always possible to do things properly "on the cheap", sometimes spending a bit more money must be done. The exception seems to be C5, hens teeth comes to mind.

The "FMV" article, I've read that again, while they are well aware of the problem, the solutions offered leave a lot to be desired. That was nearly 45 years ago, surely things have moved on since?

JasonB, yes a spacer on the crank would cause a problem as you describe, not insurmountable though. The great thing about forums, in general, is that a solution can be found if only people can get out of the "not designed here" frame of mind.

I was going to share my latest design for a small IC engine that I'm (slowly) building. Not now though, for fear of upsetting the status quo.......

John

21/12/2019 08:57:11

Some interesting comments but nothing that explains why the correct method for mounting rolling element bearings should be ignored. It doesn't matter if its an engine suitable for a first time builder or the latest very high performance engine, why not do the job properly? In most cases the design will need a little updating, why not to make a better job?

The principles have been known for many years, certainly since the Viper was designed.

Reading through the FMV engine history, those guys seem to understand the problem of different expansion rates. Not sure their solutions are good, clearly they have not heard of C3, C4 clearances.

As for my sketch, thats is what it is, a NTS sketch to show the principle, doesn't matter if there is a flywheel or propeller, principle is the same. As for which bearing (in this application) should be fully constrained, I think it best the bearing nearest the load (prop or flywheel), will be running cooler therefor less likely to want to move. I have suggested a circlip there are other ways of clamping the bearing

As for the radial fit of the bearing in its housing, I'm surprised at the somewhat casual approach to it. Again, for a successful installation its important to get these fits correct. Small diameter bearings require some very tight tolerances, always tempting to add a bit more, just in case!

Also please note that these well founded principles apply to where ever rolling element bearings are used.

My final (possibly!) word on the subject is, if at all possible, in some way acquire a copy of the SKF general catalogue (other makes are available), its all in there.

John

18/12/2019 17:10:13

I've attached a sketch of how the bearings should be mounted. This is for a rotating shaft, stationary housing. The bearings are clamped to the shaft by the nut on the end of the shaft. the order being nut, washer, flywheel (or prop driver), brg. inner race, spacer, brg. inner race and crankshaft. Note the crank shaft reduces in diameter before the end of the brg. inner race.

Hopefully it will be self evident from the sketch that there is clearance for one of the outer races to move axially, the fit in the housing would be a transition fit. The other bearing is the locating bearing. The outer race is against an abutment formed in the housing and held in position with a circlip. I've suggested a circlip but there are other ways.

Always best to use the bearing manufacturers recommendations for fits.

This arrangement will stop axial loading caused by differential expansion of the various components. Also will greatly reduce the accuracy requirements for the various axial lengths.

Look at any well designed assembly (ball bearings/rotating shaft) and this is how it will be done. If the housing rotated then the mounting would be a little different.

Over constraint of the bearings, as the are with the Nalon Viper, is a common reason for bearing failure, often mistaken for other problems.

The whole business of rolling element bearing mounting is drummed in to students of mechanical engineering, some still get it wrong or choose to ignore these basic principles with the inevitable consequences.....

John

img_20191218_163339_3.jpg

17/12/2019 08:34:32

Firstly my thanks to JasonB for pointing out that I am concerned with the axial location of the bearings.

If the (Nalon Viper) engine could be made exactly to the (poor) drawings I've studied, additional axial load on the bearings due to incorrect constraint of the bearings (over constraint) would be applied as the engine reaches running temperature.

This would be so very easy to correct and reduce the need for extremely accurate machining. If anyone is interested I will sketch the correct way to mount these bearings.

This error seems to be a common theme with this type of engine that use rolling element bearings for the crankshaft support.

Saying that its always been done this way and it works is not a reason to persist with poor engineering design. I've built several small "diesel" engines, those with ball bearings have all needed a small change to bearing mounting to correct what is a fundamental error.

Also, it looks like the crank disc of the Nalon Viper will contact the outer, stationary member of the adjacent bearing......

John

Thread: A reminder to take care with aerosols...
16/12/2019 16:36:27

Only if you smoke......

Thread: Nalon Viper
15/12/2019 08:49:25

I've just had a look at the drawings of this engine. I think there is a problem with the way the crankshaft bearings are mounted. When the engine gets hot the aluminium bearing housing will expand approximately twice that of the steel crankshaft, therefore putting additional load on the bearings.

I've looked at a few engine drawings of this type of engine and there seems to be a common theme of incorrectly mounted rolling element bearings.

Unfortunately incorrect rolling bearing mounting isn't confined to these small aero engines, there are have been many designs in our "world" that have ignored the principals. In industry the principals are well very well known but sometimes ignored. A good example is bicycles.

Definitely a need for some one to write about this in one of our magazines.

John

Thread: Foundries
12/12/2019 08:31:21

Many thanks for the replies. I was hoping for some suggestions closer to home. I like to visit a foundry to discuss details as to where runners and risers are positioned, type of sand used and so on.

I've heard good things about Manor Foundry, too far away though, likewise Hadleigh. As for Owlcastings, they need to get their website sorted, I lost patience with it!

I've considered buying melting and molding equipment but no. Time and space are at a premium, it takes time to develop the technique to produce castings of the required quality. As for melting down scrap aluminium, I want to know what I'm working with. Sometimes I have the castings heat treated.

As for multiple patterns, more work. At the moment its a small batch infrequently made. That may change in the future then multiple patterns may well be viable, preferably metal for durability.

John

Thread: Another "What is it"
11/12/2019 14:57:10

it's a fiducial indicator, used to ensure consistent readings with, most likely, micrometers used for inspection.

John

Thread: Foundries
11/12/2019 09:03:27

Good morning all. I'm looking for a foundry that can produce a batch of about 30 Aluminium castings from a single (split) pattern at a reasonable price. I'm based on the Wiltshire/Oxfordshire border. Any suggestions? I am presently using Alpro in Birmingham, reasonable castings but could be better.

I contributed to a thread some months ago on exactly the same subject. My contribution was DM Foundries at Stroud. They have just decided to shut up shop. Other foundries I have used have gone the same way. I've been in contact with other foundries, they are just too expensive. They want multiple patterns so they can produce several castings in one "box".

John

Thread: Nuts
07/12/2019 12:50:38

Theres been some mention of bolts passing through the width of the bridge. The large "washers" on this bridge are fitted to through bolts to repair and strengthen the structure. The bridge is on the London/Bristol main line. I remember it well, this bridge was on my daily commute and the road closure and traffic lights caused chaos. I felt very smug as it made little or no difference to me as I cruised by the long, long queue of traffic on my bicycle!

thru bolts.jpg

Thread: Why mostly manual cars in UK
05/12/2019 10:23:42

Back in the day 3 speed (sometimes 2 speed with a lock-up) autos did about 25mph/1000rpm in top gear. 4 speed manuals did about 30mph/1000rpm in top, therefore a significant difference in fuel consumption. This difference has narrowed considerably, but a manual will have the (slightly) better fuel consumption because the driver has complete control to drive economically.

My gearbox preference is a manual for open road sporty driving and an auto for city driving.

Slightly off topic, it surprises me just how many drivers are not really that bothered by fuel consumption.

John

Thread: Nuts
04/12/2019 16:38:16

Bridge strike!

bridge strike 1.jpg

bridge strike 2.jpg

This was a test carried out in 1996 at the Whitehouse road bridges, Swindon. These bridges, a group of 4 I think were at one time the most hit bridges in the country.

I wonder who volunteered to drive the bus?

John

Thread: Cracking a bolt
04/12/2019 08:30:19
Posted by Michael Gilligan on 02/12/2019 15:48:33:
Posted by ChrisB on 02/12/2019 14:35:32:
Posted by Nicholas Wheeler 1 on 02/12/2019 10:09:15:

ALL bolts are in tension and stretch; that's how they work!

My comment was in response to the above statement. In my opinion, bolts can be installed both in tension and shear, that's all. I was not referring to the type of joint being used - I'm referring to how forces can be transmitted to a bolt.

**LINK**

.

Thanks for the link, Chris ... I do see what you mean now

But basically what you have there is a hinge: Which is fine if you want or need to allow movement.

MichaelG.

Just had a look at the link, thats not a bolted joint, as Micheal G says its a hinge, or a pin joint i would suggest.

A bolt is never in shear, even when the bolted members are trying to slide against one another, its friction that is resisting the load. In a poorly designed and or poorly maintained joint bolts can be subject to shear unintentionally

John

Thread: Nuts
04/12/2019 07:44:07
Posted by Mike Poole on 03/12/2019 23:05:53:

Theoretically incorrect but seems to work.

Mike

Why?

John

Thread: Cracking a bolt
02/12/2019 13:35:03
Posted by ChrisB on 02/12/2019 12:47:46:

Well I have seen cases where bolts are used in shear. The bolt is retained by a castle nut which is tightened just for the purpose of not falling off and secured by a cotterpin or tab washer. All the load the bolt carries is in shear not tension in these cases I mention.

Like a lifting shackle? Not a bolted joint is it. Stretching (no pun intended) the point a bit.

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