Re: [CAD_CAM_EDM_DRO] Re: Adding inertia to an axis

One way I found to add torsional damping (for low speeds only) was simply
to use end bearings with seals- the rubber kind with the tiny steel garter
spring
that keeps the seal tight against the shaft.

A more technical description is that we are considering the leadscrew as a
torsional mechanical transmission line. We attempt to terminate that line
in its
characteristic torsional mechanical impedance.

The bearing grease seal is not a linear element-it is more of a
spring/damper
combination with a little stick slip. It does reduce torsional resonances
though.
The damping can be changed by stretching the garter spring a bit.

Les

Leslie Watts
L M Watts Furniture
Tiger, Georgia USA
http://www.rabun.net/~leswatts/wattsfurniturewp.html
engineering page:
http://www.rabun.net/~leswatts/shop.html

----- Original Message -----
From: "Jon Elson" <elson@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Thursday, May 30, 2002 1:39 AM
Subject: Re: [CAD_CAM_EDM_DRO] Re: Adding inertia to an axis


> Peter wrote:
>
> > In the recent thread about X-Axis won't do as it's told - there was a
> > suggestion from Hoyt to try adding inertia to the axis. As it turned
> > out that wasn't the problem in Richard's case - but it may be the
> > problem for one of my machine axes. But I'm curious as to how much
> > extra inertia to try adding... I'd like to start in the right ballpark.
> >
> > I have a NEMA42 motor so I will probably try turning a solid steel disc
> > around 3" dia and say 1" long as a starting point, recognizing that the
> > disc's OD is most significant in moment of inertia calculations. Would
> > appreciate any suggestions other than my wild guess as a starting point.
>
> Adding true inertia only moves the resonance to a lower speed. that may
> not help at all, as it may intensify the resonance by adding more mass to
> vibrate. The professional approach is a viscous damper. Early ones
> were made by attaching a disc to the shaft, and clamping a heavy metal
> disc to that one with a rubber sheet between. With the rubber sheet
loosely
> clamped, the attached disc would rub against the dead weight and absorb
> the vibrational energy. getting the clamping pressure right, and keeping
it
> right was not easy. The current approach is to mount a hollow round
> cavity to the shaft, and have a heavy metal disc free floating in the
cavity.
> The cavity is partially or completely filled with a viscous oil to give
the
> desired vibration absorption. I would think it wouldn't be too hard a
> lathe project to make one of these out of aluminum, with a steel disc
> inside, and a cover plate with a gasket. SAE 90 gear oil should provide
> good damping.
>
> Jon