Rotary thrust actuator, How to make?

CalG · Mar 30, 2014

The application is a friction drive.

Simple plate on a shaft pressed to a second driven plate with a friction surface. Much like any automotive mechanical clutch, just not a spring loaded pressure plate "sandwich".

The shaft wise actuator is a three lobe roller ramp, concentric with the mainshaft. A simple radial lever rotates the actuator, the ramps give a shaft wise linear motion to the drive disc, pressing against the driven friction plate and so driving the disc.

Without a dividing head gear trained to a milling machine feed, how would one produce such a "trust bearing cam" suitable for roller between the contacting elements? (hard surfaced and rather precise, say +/- .005 between lobes)

General scope:

5 mm of liner motion with 45 degrees of rotation
carrying shaft diameter about 35 mm.

"agricultural" application,

Bottom line: a friction drive actuator in the least amount of linear space. SIMPLY! There is no room for forks and throw out bearings, plus the direction of actuation is 90 degrees out if the "conventional automotive practice is considered.

If I had a picture of such a mechanism, I would include it.

No need to consider the stationary and rotating parts, that's all covered. 'Just how to make those ramps....?

Ideas?

TIA

ps I can see mounting the work in a dividing head and then "stepping" out a ramp profile, then smoothing the results, harden the working face, followed with some careful work with a slip stone. There must be a better way ;-)

Milland · Mar 30, 2014

CalG said:
The application is a friction drive.

Simple plate on a shaft pressed to a second driven plate with a friction surface. Much like any automotive mechanical clutch, just not a spring loaded pressure plate "sandwich".

The shaft wise actuator is a three lobe roller ramp, concentric with the mainshaft. A simple radial lever rotates the actuator, the ramps give a shaft wise linear motion to the drive disc, pressing against the driven friction plate and so driving the disc.

For a very simple method, use a large ball endmill to create a radius ramp of sufficient height. Perhaps a 1" ball will give a reasonable ramp rate with ~.2" of profile geometry (too small a ball cutter will give an unsatisfactory rate of rise). Orient the cutter path such that the "active area" (preferential ramp surface) will be perpendicular to the mating surface, if that's an issue.

Something I've done in similar circumstances is to use a smaller ball endmill to cut a short helical path into two end faces of a cylinder, then use captured ball bearings that would follow the ramp helix as one part was rotated. Very low friction (but low load capacity too) for a given travel. If it was a high-force application the parts could be heat treated after helix cutting, the "milled in" accuracy would usually be good enough without additional work.

S_W_Bausch · Mar 30, 2014

The old-school tactic was to plot one out, roughly, then create it, roughly, and use it as a master pattern.

If you would tolerate a sine curve, easy enough to generate one of those on graph paper, an Excel spreadsheet, etc. and then step out the profile as you suggested.

You could also rotate a cylinder whose axis spun a geartrain that rotated a crank, that moved a connecting rod, that moved a link/piston axially to the cylinder. That gear train would either be 1:1, or an integer reduction ratio such as 2:1, 3:1 etc.

I have collected the Kearney Trecker pamphlet on the subject, as well as some articles that first appeared in Machinery Magazine (If I recall correctly). I should still have them...

You could have a CNC plasma, water jet, torch etc. cut the sine on a strip, then have it rolled, but I don't know if I would do it that way.

Cam Design and Manufacturing Handbook, Robert L. Norton.
Kearney Trecker Methods of Cam Milling.
Designing and Cutting Cams, Louis Roullion, Arthur B. Babbitt, F.H.Sibley, James L. Dinnany.

CalG · Mar 30, 2014

Ahh! This is a job well suited to a shaper! ;-)

I can see that "stepping off the profile" with the work mounted in the dividing head should answer for this application. Some small diameter milling cutter on the Horizontal should work fine.

Crank the DH, lower the table...crank the DH, lower the table....lather, rinse, repeat ...;-)

Sometimes just putting the idea down on "paper", getting and then getting response is enough to light the light...dim as it is ;-)

Billtodd · Mar 31, 2014

Mount the cam part at the helix angle on an RT . Mill the cam face by rotating the table 45°. Return table to zero , index part ...

It may not produce a completely linear ramp , but would be repeatable.

Bill

TGTool · Mar 31, 2014

Billtodd said:
Mount the cam part at the helix angle on an RT . Mill the cam face by rotating the table 45°. Return table to zero , index part ...

It may not produce a completely linear ramp , but would be repeatable.

Bill

When I read the description I was picturing a vertical rotary table, 45 degrees to the X axis and milled using Y travel. Again, a kludge but repeatable. This presumes no clearance problems following the curve as the depth changes.

S_W_Bausch · Mar 31, 2014

There is at least one Youtube video of cam milling.

APD · Mar 31, 2014

Im not sure if Im totally clear on the design constraints, but could the back of the friction plate on the drive side be dish shaped or ramped?...so that the radial position of the actuating lever determines the amount of linear movement.

CalG · Mar 31, 2014

There is this one
Cam Milling - YouTube

A useful method for a radial cam profile and would be adaptable to a face profile cam with a simple orthagonal orientation of axis.
A procedure that I have read about and keep a reference (Not having a screw machine, No need has come up yet ;-)

But of course, as the original post alludes to :
"Without a dividing head gear trained to a milling machine feed, how would one produce such a "trust bearing cam" suitable for roller between the contacting elements?"

Ahh, if only we all had 4 axis HMC's in the shop ;-)

CalG · Mar 31, 2014

APD said:
Im not sure if Im totally clear on the design constraints, but could the back of the friction plate on the drive side be dish shaped?...so that the radial position of the actuating lever determines the amount of linear movement.

APD

That is the desired mechanism. By rotation position of the actuator arm over some angle less than 45 degrees, the linear motion would be accomplished.
At this moment, I do not see how "dishing" the drive side would be significant, though it could allow the width of the actuator lever determine the effective length of the mechanism on the central shaft. With the face cam and roller element volume being accomodated within the dish. Nothing wrong with that, though it does complicate the details of the friction plate.

I realise the method I am looking for might also produce a certain type of one way drive coupling. With the "camming out" profile providing the linear motion desired.

Like this
http://www.coolspringpowermuseum.org/Exhibits/Jargon/Jargon_files/Face_Cam.JPG

But perhaps refined enough to accept rollers between the two parts to reduce friction ;-)

This:
http://www.chinhao.com.tw/userfile/gallery/pictures/200962416464375214.jpg

But no key in the bore (floats on the central shaft). and add a radial lever arm ....

for the adventurist
http://farm6.staticflickr.com/5097/5401857354_1ab0995d2f_o.jpg

cheers

S_W_Bausch · Mar 31, 2014

Well, it's either a dividing head to replicate the sine curve, or a gear increaser and crank to replicate the curve.

Actually, the crankshaft (and connecting rod) introduces some distortion into the curve, but it's close.

This challenge is simply a variation of "How to Cut Oil Grooves?" and the old-timers were willing to take an old lathe and modify into a single-purpose machine:

Adapting an Old Lathe for Cutting Oil Grooves.jpg

Adapting an Old Lathe for Cutting Oil Grooves

Shops handicapped by not having a regular oil-grooving machine, can arrange an old lathe to do this work acceptably.

After the apron has been removed from the carriage, a special swivel block is attached to the rear.

The lead screw is taken off, and in its place a small shaft is substituted.

At the end of this shaft is a miter gear, meshining with a similiar gear on a short shaft at the end of the bed.

In the case of a very long machine, this shaft could be put through holes made in the bed.

On the back end of this shaft there is a slotted disk, similiar to a disk used on a shaper or slotter, so that different lengths of stroke can be obtained.

A connecting rod runs from the crank pin to the swivel block on the rear of the carriage.

The length of oil groove cut depends on the throw of the crank, which is varied by moving the crank pin out or in.

The change gears are provided so that the pitch of the groove may be varied.

For example, if the crank makes one revolution to one of the spindle, the resulting groove will be a loop, not crossing itself at any point.

If the spindle is geared to make two revolutions to one of the crank, the groove will cross itself at one point, in the center of the length of the groove.

By providing play between the adjustable collars, where thay make contact with the swivel block, the tool will dwell at the end of the stroke, thus making an additional variation in the shape of the groove.

The contour of the groove is governed by the shape of the tool, which must be ground with plenty of clearance. If desired, a special rigid tool block may replace the compound rest.

Credited to H.L. Wheeler.

CalG · Mar 31, 2014

SW

Thanks for the examples and sketch work.

In consideration of maximisation of active surface, a sine wave gives away half of all that is wanted in the "down hill slope".

Just a rough "mental sketch" using the DH and the stepping procedure.
If a single cam provided .120 of lift in 120 degrees of arc (three lobes = 360 degree) That would be .001 per deg. A pretty simple relationship to set up.
But I suppose some allowence would need to be made for the "dwell" between the end of one lobe and the begining of the next in order to satisfy the space requirements of the milling cutter.

The inner diameter of the cam with respect to the milling cutter diameter would set the limit!

johnoder · Mar 31, 2014

You need conical faces and conical rollers if you can't accept "skidding" of the rollers on the face of the ramps - for the same reason road vehicles have differentials

CalG · Mar 31, 2014

skidding is acceptable.

Conical surfaces would complicate matters. Jeesh! Does a fellow(s) need a gear shaper for such a simple part?

snowman · Mar 31, 2014

just use a slip clutch, with teeth every 45 degrees...

Meaning, instead of cutting a cam, cut two mating slip clutches, then weld them on to a shaft.

Like this...with less teeth...which you could cut with the indexer of your choice held at an angle....you aren't worried about all the center crap, just the outer diameter...so you can have cutter overlap in the center.

snowman · Mar 31, 2014

Just do it the really easy way...45 degrees is 5 turns on an 40:1 dividing head. That's two inches (50.8mm)....if you put a 1:1 timing belt on the bridgy x.

A 1:10 belt ratio will get you 5.08mm (0.2"), within or 0.004" of 5mm (0.196")...theoretically of course.

dsergison · Mar 31, 2014

you want to make what looks like this correct?

I did not hear that you want to be able to go smothly over-center or the wrong way to activate.. so jagged teeth make the most use of your range of motion.

just tip your mills head, or tip your indexer, and cut.
not perfect helical geometry, so what. make the ramp portion relatively narrow comapred to the diameter.

if you cut by rotating the rotarty table it will be very nearly the correct geometry.

johnoder · Mar 31, 2014

That's two inches (50.8mm)....if you put a 1:1 timing belt on the bridgy x.

Five turns is 1.000" since the screw is 5 pitch

snowman · Mar 31, 2014

I thought in my head 200 thou, but somehow, multiplying it by 5 worked out to 2 inches.

wow

9100 · Mar 31, 2014

If you can tolerate a radius at the bottoms of the teeth, I could just put it on the 4th axis of my CNC mill, which is horizontal, bring an end mill up to it, type in the angle and rise, and let it do its thing. With some effort I could tilt the 4th axis and generate the appropriate taper to use with tapered rollers or better, use a tapered cutter. If you want it hardened and ground, I have a Precise high speed grinder that mounts on the mill head.

Bill

Rotary thrust actuator, How to make?

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