ET: Engine (Twin) Camshaft design

[Deleted member 3831]

Re finger followers, I note in MPH 681, page 51, that a reproduction is given of the Vincent radius follower, which also is dated as 1953.

This is a sketch made by Jim Smith on a piece of paper and dated 1951. It would appear that Jim had a standard follower built up with stellite or similar, and then ground to the given radius

 

[passenger0_0]

Grey One,

Not Ariel, rather a custom design based on the same leading/trailing arm follower system used by Vincent.
This was a custom side-valve (flathead) application for a 1928 1200 cc Indian Chief Land Speed attempt that has yet to hit he salt.
0.500" lift inlet - 312 degrees duration
0.450" lift ex - 285 degrees duration
Followers and cams were CNC machined and have pressure oil jets directed to all sliding surfaces.
Single coil bee-hive valve springs set to 70 lb seated pressure.
We're limiting engine speed to 6,000 rpm with respect to the high piston speed and long con rod and have had no indications of valve float.

 

[Bill Thomas]

Re finger followers, I note in MPH 681, page 51, that a reproduction is given of the Vincent radius follower, which also is dated as 1953.

View attachment 18976

This is a sketch made by Jim Smith on a piece of paper and dated 1951. It would appear that Jim had a standard follower built up with stellite or similar, and then ground to the given radius

Would a standard follower work with Somerton type cams ' And if so, What sort of performance, I didn't think it would work. Cheers Bill.

 

[Deleted member 3831]

Grey One,

Not Ariel, rather a custom design based on the same leading/trailing arm follower system used by Vincent.
This was a custom side-valve (flathead) application for a 1928 1200 cc Indian Chief Land Speed attempt that has yet to hit he salt.
0.500" lift inlet - 312 degrees duration
0.450" lift ex - 285 degrees duration
Followers and cams were CNC machined and have pressure oil jets directed to all sliding surfaces.
Single coil bee-hive valve springs set to 70 lb seated pressure.
We're limiting engine speed to 6,000 rpm with respect to the high piston speed and long con rod and have had no indications of valve float.

I wish you every sucess with your Indian project.

It seems the spirit of Burt Munro is still very much alive in the land of the long white cloud.

 

[timetraveller]

There was a special Picador cam. Some Picador engines used Mk II cams and others had the specials which have to be used with radiused followers. My understanding, and clearly you have more experience with this than I have, is that the combination of a flat lever cam follower and any practical cam cannot give a correct lift/acceleration curve. I had convinced myself that it was impossible but earlier you showed that it is possible in principle but, if I understood correctly, not in practice. The Picador engines were reputed to have been designed to run at 6,000 rpm continuously and at the time someone was bright enough to realise that a better cam design could be made for that.
I have never heard of, and cannot imagine why, anyone would want to run a flat follower cam on one valve and a radiused follower cam on the other.

 

[timetraveller]

A reply to Bill, #23 above. You cannot use a flat follower with the Picador style cams, Just one reason, The Picador cams have a hollow flank into which a flat follower go. There are many other reasons as well.

 

[Deleted member 3831]

I have to refer back to my post where I wrongly assumed the models passenger0_0 had posted were Ariel cams and followers.
My explanation why a flat follower might be used for one cam and a radius for the other has indeed occurred with Ariels, and the reason is that subtle variations can be made this way, sometimes when using one cam lobe to fulfil both valve lift requirements.
I did an exercise some years back for members of the Ariel club, identifying and creating data files so that in future the club has data that will enable any of the cam forms to be re manufactured. During this exercise I was shown just how many different cam and follower combinations were possible, flat, radius, high lift. low lift, the permutations were considerable.
Time traveller,
you say the Picador profile has negative flank curvature, so this being the case you would not want to be rash and mate it with a flat follower, but there are many cam forms in other engines which have no negative component, yet are working with a radius follower. To use a flat follower as a replacement will automatically have the effect of increasing the area under the lift curve, while leaving the maximum valve lift and the duration unchanged.
Passenger0_0,
I have never seen an Indian side valve engines cam followers, but am much more familiar with Burt Munro's OHC conversion. Burt made his own cams using a jig he erected on his Myford lathe, from masters he made that were the inverse of the resultant profile. Sad to say, Burt's lathe which was used as a feature in the film, went missing after filming was completed, and as far as I know has still not been found.

 

[Deleted member 3831]

To add further to the queries raised re negative curvature , I can best illustrate by referring to Rudge Whitworth's parcipitaation in racing at the 1928 Isle of Man TT races.
Their rider, Jack Amott, used cams to the form as illustrated, matched up to a finger follower of 0.53" radius.
Note the drawing indicates a cam form comprising two circles, base and nose, connected by tangents.

The blending base circle was smaller than the true base circle, and a blending radius was used to connect smoothly between the tangent and the base circle radius, creating a short period of negative radius.

The 0.53" radius of the follower was necessary for use with this cam form, but some years later, the base circle radius was constructed as a positive radius, and with the resulting cam form both flat and radius followers could be used

 

[passenger0_0]

Interesting conversation.
As rightly pointed out , it isn't practical to use flat followers when using serious lift and duration cams as the point of contact 'jumps' from one end of the follower to the other so you loose your sliding contact and encounter wear issues.
The Indian cam system I mention also used a concave form, necessary to get symmetrical lift profile, and was on the acceleration limits as defined by AG Bell.
No matter how you look at it, when using pivoting cam followers, you always end up with the point of cam & follower contact moving all over the place so you end up with strange looking asymmetrical cam profiles to realise symmetrical valve actuation.

 

[Phil Davies]

As I understand things, the Picador cam profile was designed in partnership with the radiused followers to give the cam follower a lesser stressed life at the much longer duration 'high revs' the Picador was required operate at (especially with the triple valve springs used).
Lever followers of the road engines are highly stressed at high revs (as I found out when racing) and were never stress tested properly even with the lightning engines as they never had to run flat out for long enough compared to what was asked of the Picador.
The Picador cam and follower form was (I believe) developed to follow what can be seen as a polydyne form that allowed the proper distributed loading of the follower.
As others have detailed, the radiused follower can only be used with a sympathetic cam form and a polydyne cam form does not work with a flat follower.