Author Topic: Camshafts and Timing  (Read 8257 times)

Online Roger B

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Camshafts and Timing
« on: June 02, 2014, 06:47:01 PM »
Am I confusing myself?

When you design a camshaft (for example using the late Ron Chernich's (I hope he is in engine heaven) cam calculator) this does not take any valve clearance into account. If I look at various suggested cam timings (for example in Malcolm Strides book) I assume these are also pure cam designs without the clearance. The clearance significantly reduces the opening angle of the valves, more so in our models where the clearance is a significant proportion of the total lift.

I was expecting an opening angle of around 250°, I can measure around 220°. This is certainly where the planned overlap in my first cam disappeared  ::) . How do others compensate? Does it really matter???
Best regards

Roger

Offline gbritnell

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Re: Camshafts and Timing
« Reply #1 on: June 02, 2014, 08:00:56 PM »
Roger,
When you develop the specs for a camshaft and then do a layout for machining it you are only concerned with when the lifter starts to open the valve without the clearance. Granted what you say is correct, the greater the clearance the later the timing is but for our model engines it's not that critical. If you really wanted the valve timing spot on then you would have to take the clearance into account. Usually for my small engines I use .002-.003 valve clearance. I base this on my 400 cc. motorcycle engine which has .004 intake and .006 exhaust. I doubt that a valve with a .25 diameter head and .078 valve stem would grow that much in operation and the .002-.003 clearance won't affect the total lift.
Automotive cam timing is generally set at .050 lift. That's not to say that that's what the clearance is but rather an easier number to read then just when the valve is starting to open and this is what the cam manufacturer has on his timing sheet when he sells you the cam.
gbritnell
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Offline lakc

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Re: Camshafts and Timing
« Reply #2 on: June 02, 2014, 11:51:27 PM »
Your spot on Roger.
Its Georges fault, that he has not done a post on making teeny tiny hydraulic lash adjusters.  :Jester:
But the usual method of compensating is to make the cam then undercut the base circle by the anticipated lash. That will have to do until George stops being lazy.  :)
Jeff

Offline stevehuckss396

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Re: Camshafts and Timing
« Reply #3 on: June 03, 2014, 12:57:47 AM »
Your spot on Roger.
Its Georges fault, that he has not done a post on making teeny tiny hydraulic lash adjusters.  :Jester:
But the usual method of compensating is to make the cam then undercut the base circle by the anticipated lash. That will have to do until George stops being lazy.  :)


Hahahhhahahahhaaaaaa!!!!!    If anybody can make tiny Hydro lifters it's George!


I believe Ron's Camcalc program was not intended to be a design tool. It's function is to generate a table to cut a camshaft with a set of specific input data. The design should be done before you use the program to calculate the data to cut it.

How do others compensate?  Put more duration in your cam lobe profile. Personally I put about 280 - 300 degrees in my cams and lash .003ish and it is what it is. Of course this is for a multi-cylinder engine and not a hit miss.

Does it really matter?  Maybe. It does to me. I think an engine with more duration has a rich, warm sound to it at idle. If you want an engine that runs and it doesn't matter what it sounds like, then probably not.  If you are worried about 220 Vs 250 I am not sure you will be able to see or hear a difference in a model engine.
Do not be like the cat who wanted a fish but was afraid to get his paws wet.

Offline Graham Meek

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Re: Camshafts and Timing
« Reply #4 on: June 03, 2014, 09:53:55 AM »
Hi Roger,

When I made the cams for my Seagull engine, (there is an article on Ron's site, bless him), I undercut the base circle to compensate for the tappet clearance. The undercut on the base circle runs out at the flank / base circle intersection point so it does not effect the cam opening times. This technique also helps when grinding the profile which is easy to do using my approach.

While Edgar T Westbury and Professor Dennis Chaddock used to specify a 0.005" tappet clearance, the Seagull is set to run on 0.002-0.003" tappet clearances. These were reset after the initial running-in and have not been touched since, the engine has now clocked up over 80 hours running so far. This engine regularly runs for over an hour at a time as it is a great source of amusement for my Grandson, he likes to watch the thermostatically controlled fan cut in and out as well as tell me to top the tank up with fuel.

My best regards
Gray,

Offline tangler

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Re: Camshafts and Timing
« Reply #5 on: June 03, 2014, 11:04:17 AM »


I believe Ron's Camcalc program was not intended to be a design tool. It's function is to generate a table to cut a camshaft with a set of specific input data. The design should be done before you use the program to calculate the data to cut it.

How do others compensate?  Put more duration in your cam lobe profile. Personally I put about 280 - 300 degrees in my cams and lash .003ish and it is what it is. Of course this is for a multi-cylinder engine and not a hit miss.

Does it really matter?  Maybe. It does to me. I think an engine with more duration has a rich, warm sound to it at idle. If you want an engine that runs and it doesn't matter what it sounds like, then probably not.  If you are worried about 220 Vs 250 I am not sure you will be able to see or hear a difference in a model engine.

While Camcalc is primarily for cutting cams, the derived acceleration figures have a design function in enabling calculation of the spring force required to keep the valves on the cam.

I'm intrigued by the perception that longer valve openings give a nice sound at idle.  Longer valve open times usually means more overlap - inlet and exhaust open at the same.  The overlap takes advantage of the inertia of the gasses to ensure optimal combustion and scavenging at speed.  This suggests that at low speed the sound you like is of an engine running rough  :o  But I think I know what you mean - the sound of an engine popping and rumbling at idle gives an impression of constrained power.

Cheers,

Rod

Online Roger B

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Re: Camshafts and Timing
« Reply #6 on: June 03, 2014, 11:53:50 AM »
Thank you for all the comments  :ThumbsUp:  :ThumbsUp:

I set the valve clearance to 0.005" when I put the engine back together. I could possibly reduce this to 0.002 - 0.003.

George, I remember using a much larger valve clearance when setting the cam timing on my full size engines. This meant that you were really measuring when the valve was effectively open. There won't be much gas flow as it just comes off the seat.

Steve, you are using a much bigger opening angle. What angle do you use between the lobes?

Gray, did you do any special blending between the reduced base circle and the lobe? I would have to do a few drawings to see what the effect would be when milling the profile.

I checked the cam timing on my horizontal engine in the same way as I did for the vertical and got almost the same results. Although I could increase the angles it appears to be a workable design. Back to looking at the carburation again   ::)  :headscratch:
Best regards

Roger

Offline gbritnell

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Re: Camshafts and Timing
« Reply #7 on: June 03, 2014, 11:58:23 AM »
Hi Roger,
When you say "back to looking at the carburetion", are you having particular problems with your engines?
gbritnell
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Offline Graham Meek

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Re: Camshafts and Timing
« Reply #8 on: June 03, 2014, 01:07:33 PM »
Hi Roger,

I used a cutter that was about 0.75 of the diameter of the base circle, this gives a nice blend radius so that the follower does not hit a distinct ramp or edge, as would be the case if you used a smaller diameter cutter. The attached photograph is of a friends Chenery camshaft built using my method, if you look closely at where the ground flanks blend into the base circle there is hardly any discernible transition. Although this engine design was heavily modified by me to run on glow plug it was a very smooth engine.

My best regards
Gray, 

Online Roger B

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Re: Camshafts and Timing
« Reply #9 on: June 03, 2014, 01:24:06 PM »
Hi Gray,

I'm not sure I quite understand. Was this cutter co axial with the camshaft or at right angles? You also mentioned grinding. Do you have any pictures?

Hi George,

Somewhere between problems and learning  :insane: The engine is my own design so I am trying to understand and if possible optimise as I build.

http://www.modelenginemaker.com/index.php?topic=2093.105

It runs, but currently not very well. I currently am aware of some problems with the carburation which will be my next steps. Phil (Philjoe5) mentioned that he had problems with valve timing on his engine so I decided to check that as well, hence this post.
Best regards

Roger

Offline Graham Meek

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Re: Camshafts and Timing
« Reply #10 on: June 03, 2014, 04:13:41 PM »
HI Roger,

Yes you are right, I do my cams side on, the Flank radii are turned in the lathe. If a suitable amount of material is "left on" then the flanks can be ground after hardening & Tempering as all machining is done reference the keyway. The base circle is machined as well as the nose radius on the second jig again reference the keyway. As Edgar T Westbury used a lot of common cam profiles and sizes one set of jigs will do several engines.

The article "On Cams made easy" appeared in HSM at the beginning of the year and it is also on Ron's site.

My best regards
Gray,

Offline lakc

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Re: Camshafts and Timing
« Reply #11 on: June 03, 2014, 05:28:29 PM »
Roger,
In the past I have just undercut the base circle. I milled my cams end on and it tended to blend quite naturally. I did look up my new spreadsheet for calculating camshafts, and see I included lash and duration adjustments in my new formula. I would post it here, but it was based of another spreadsheet, and should make an effort at least to obtain permission, or at least be able to give attribution to the original work. (dont remember where I got it from??)
So, your answer is I am including the lash specs into the final duration measurement, therefore the actual cam duration is greater then advertised.
My next IC project will be a cam grinder.
Jeff

Online Roger B

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Re: Camshafts and Timing
« Reply #12 on: June 03, 2014, 05:30:48 PM »
Hi Gray,

thank you for those pictures, it makes sense now. How do you fix the lobes to the shaft, with Woodruff keys?
Best regards

Roger

Offline Graham Meek

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Re: Camshafts and Timing
« Reply #13 on: June 03, 2014, 06:04:37 PM »
Hi Roger,

The cam is drilled from the back of the base circle 1.5 mm, (1/16" approx), this lines up with and is the same size as the keyway. The Embryo camshaft is drilled to take the roll or dowel pins if preferred, on the Seagull camshaft this only involves two index movements about the central timing gear location dowel. The second bank of cams is a mirror image of the first so each centre line is common.

This method means you can change your cam overlap simply by drilling a new shaft, and salvaging the old cams by drilling out the roll pins.

The Chenery camshaft uses a similar arrangement but with short location pins locating the cams via the keyway, there is no hole in the back as spacers between the cams fixed with Loctite locate the cams laterally. This was the earlier version of the above method which is much easier to assemble and dismantle if needs be.

My best regards
Gray,

Online Roger B

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Re: Camshafts and Timing
« Reply #14 on: June 03, 2014, 06:17:57 PM »
Hi Gray,

Thank you, as ever, for the explanation and pictures (generally I need pictures to understand)  :cheers:

Jeff,

Thank you for your replies. I will look at modifying my spreadsheet to reduce the base circle for the next camshaft. I suspected that the final polish would remove most of the problems.
Best regards

Roger

 

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