In article ,
"Buerste" wrote:

"Joseph Gwinn" wrote in message
...
In article ,
"Buerste" wrote:

"Tim" wrote in message
...

"Buerste" wrote in message
...
I have some air circuits that are controlled by cams that actuate
manual
air valves that look like limit switches with roller arms that follow
the
cam. Would I be better off with electric limit switches on the cams and
use electric air valves? The cam rotates at 120 rpm. The dwell on the
switch is 45 degrees.

I do have to replace the air switches every 2 years or so.

I will NOT install a PLC or anything more complex.

I have a similar application coming up and will need to use electric
relays to control air valves or air piloted valves to control air
valves. Which is more reliable? I do have really clean, dry air.

Bill's post made me think of this.


Have you carefully considered the profile of your cam? Adjusting things
for minimum movement, and ramp angles the minimize the speed of the
roller
arms could help.

How many cycles do you make in two years? What is the pressure and
diameter of the air line. I would think a roller tip micro switch and a
quality relay should last for millions of cycles.


Hmmm, I'd say an average of 28,800,000 in 2 years. Air line is 1/4"
Nylon
at @100 psi.

How accurate and repeatable does the trip point need to be?

I'd be tempted to use an aluminum cam sensed by an electromagnetic
non-contact proximity sensor of some kind. This will work through most
kinds of dust and dirt, and can be hosed down as needed.

What is the dust and dirt mainly made of?

Joe Gwinn

Dust is wood dust from drilling one 9/32" x 1/2" deep hole in Beech every
rotation. The cam is "soft" activation and the trip point is +/- 10 deg or
so.

This ought to be easy. Wood dust will not affect an electromagnetic
prox sensor at all. A cam with sharp ramps will easily manage ten
degrees. The absence of contact means that the cam profile need not be
that gentle.

A cam made of aluminum with some kind of adjustable collet lock on the
shaft ought to work just dandy with a prox sensor with circuit closure
output.

One can buy adjustable ready-made cams where one can set the duty cycle
by adjustment of the cam halves. Perhaps one of these will suffice.
Failing that, one can always cut a cam with a scroll saw. As there is
no physical contact, the surfaces need not be smooth.

Joe Gwinn