How much can motor feet vary from being in a perfect plane? Have you ever measured motor feet and how can you do it with reasonble efforts?

Foundation mounting surface flatness. How much can a foot be allowed to be out of a perfect plane? Do you test the feet before installing the motor?

Does above really have an importance for long term life span of a motor and coupling rubber blocks etc.

I am testing to add to the purchasing specification that feet contact surface should be flat with deviation from perfect less than 20 percent of the motors airgap if the motor is a normal induction 2, 4 or more pole design. Does that make sense? Or should there be a fixed number disregarding the motors size?

This message has been edited. Last edited by: Arne Lindholm,

The real answer I don't know. Here's all I know about it:

Some ways to measure feet from worst/easiest to best/hardest:

1 - Meaure coupling movement when loosen a foot.

2 - Measure top of foot movement when loosen a foot.

3 - Measure all around under the foot with a feeler gage while motor sits on it's base.


Some standards:
NEMA MG-1 FooT coplanarity
4.15 – Method to check coplanarity of feet
Pput motor on flat surface (tool grade B). Feeler gage of required tolerance shall not penetrate a circle centered on bolthole with diameter = 1” or 3xBolthole diameter, whichever is greater. Table 4.4.1 Notes give required tolerance
(1) cast iron – bottom of feet is 0.015”
(2) cast iron – foot bottom parallel to foot top within 1.5 degree.

IEEE841-1994 Frame requirements including foot flatness
1. Section 6.3 a, c, d, e.
a) foot flatness requirements.
(1) 5 mil under foot on granite table tool room grade B (relatively high)
(2) <1.5 degrees on top of foot (so bolt head is resting on a flat surface).

Percent airgap is an interesting approach. Actually I don't think airgap varies that much among induction motors. Somewhere between 60 and 150 mils (that's a US mil, not a metric one) for all shapes and sizes as far as I know. If I wanted to put a tighter tolerance on some motors I would put it on the 2-pole motors. Those are the ones that can create quite a problem if feet are not exact.

A hint when you buy 2-pole motors, make sure they are tested rigidly mounted. NEMA MG-1 allows resiliently mounted and that is often the way they are done with small motors. The irony is that NEMA actually gives you a higher spec for resilient mounting but the 2LF can be much higher with rigid mounting.

The real answer I don't know. Here's all I know about it:

Some ways to measure feet from worst/easiest to best/hardest:

Meaure coupling movement when loosen a foot.

Measure top of foot movement when loosen a foot.

Measure all around under the foot with a feeler gage while motor sits on it's base.


Some standards:
NEMA MG-1 FooT coplanarity
4.15 – Method to check coplanarity of feet
Pput motor on flat surface (tool grade B). Feeler gage of required tolerance shall not penetrate a circle centered on bolthole with diameter = 1” or 3xBolthole diameter, whichever is greater. Table 4.4.1 Notes give required tolerance
(1) cast iron – bottom of feet is 0.015”
(2) cast iron – foot bottom parallel to foot top within 1.5 degree.

IEEE841-1994 Frame requirements including foot flatness
1. Section 6.3 a, c, d, e.
a) foot flatness requirements.
(1) 5 mil under foot on granite table tool room grade B (relatively high)
(2) <1.5 degrees on top of foot (so bolt head is resting on a flat surface).

Percent airgap is an interesting approach. Actually I don't think airgap varies that much among induction motors. Somewhere between 50 and 150 mils for all shapes and sizes as far as I know. If I wanted to put a tighter tolerance on some motors I would put th tighter tolerance on the 2-pole motors. Those are the ones that can create quite a problem if feet are not exact.

A hint when you buy 2-pole motors, make sure they are tested rigidly mounted. NEMA MG-1 allows resiliently mounted and many manufacturers do that. Then your 2-pole 2LF problem might not show up until you bolt it down in the field.

Pete,
Is that coplanarity of feet at .015" or .0015"?
Arne, that is a different approach, but I'm not sure that I would accept up to 20 percent. I can't say for sure, because I never check the motor feet, but most of the softfoot problems have been considered a base issue. Who knows, I may have been shimming out a motor issue!

0.015" is what NEMA says. It sounds awfully high to me. 0.005" (IEEE841) sounds a lot better. With the other method - dial indicator on top of foot, don't most folks look for <=0.002" change when tightening/loosening?

Hi Arne,

It should be a fixed 'max' number in my opinion. But it sounded like you are after a new motor specification for a new purchase rather than a field installation acceptance tolerance????

At what specification should the motor's feet be mill to? In the same plane and parallel to the motor's concentric centerline of the bearing bores +/- 0 deg 0 min 15 sec not to exceed 2 mils. Any good machine shop should be able to meet this; else get a better shop or motor manufacturer.