Maximum allowed RPM
11-14-2013, 01:44 PM,
 shantanu Senior Member Posts: 88 Threads: 32 Joined: May 2011 Reputation: 1
Maximum allowed RPM
Hi all,
Please let me know the maximum allowed RPM of a part on which thermal spray coating has been done for dimensional restoration . Coating may be HVOF or twin ARC. Coating has been done to rebuild the journal bearing seat

Regards

Shantanu
11-14-2013, 09:22 PM,
 djewell SuperMember Posts: 239 Threads: 7 Joined: Jan 2008 Reputation: 6
RE: Maximum allowed RPM
You should choose your RPM to achieve a surface speed of 1 - 1.5 m/s. You can't say X is the maximum RPM for every part. Calculate surface speed using the formula Velocity = radius * omega.
11-15-2013, 05:07 AM,
 shantanu Senior Member Posts: 88 Threads: 32 Joined: May 2011 Reputation: 1
RE: Maximum allowed RPM
Hi Djewell,

Let me rephrase my question:

If a part is rotating at a RPM of 12,000-15000, can its journal bearing seat be built by a thermal spray process like an ARC or HVOF or shall the force created during such high speed rotation lead to peeling off of the coating.....

Basically, till what maximum ROM in working condition can a part be rebuilt using an ARC or HVOF process

Regards

Shantanu
11-15-2013, 10:25 PM,
 djewell SuperMember Posts: 239 Threads: 7 Joined: Jan 2008 Reputation: 6
RE: Maximum allowed RPM
I have never heard about a limitation on part rotational speed in service. In the case of a coating on a bearing journal, the bearing will keep the coating from coming off. Technically, if the centrifugal force exceeded the bond strength of the coating and the cohesive strength of the particles in the coating, it could fail.
11-16-2013, 02:56 AM,
RE: Maximum allowed RPM
Hi Shantanu

What is the diameter of the part? RPM alone will not tell you the surface speed or the forces acting at the surface. I have sprayed parts with surface speeds up to 10 m/s using plasma and HVOF with no coating problems. The smaller the diameter the higher the RPM needed.
11-16-2013, 05:04 PM,
RE: Maximum allowed RPM
Hi Shantanu

I think I interpreted your question wrong. If this about forces acting on a coating during service, then my comment on the diameter being important stands. Also important will be the mass of the coating supported by the weakest point, usually the coating/substrate interface.
11-18-2013, 09:54 AM,
 shantanu Senior Member Posts: 88 Threads: 32 Joined: May 2011 Reputation: 1
RE: Maximum allowed RPM
(11-16-2013, 05:04 PM)Gordon Wrote: Hi Shantanu

I think I interpreted your question wrong. If this about forces acting on a coating during service, then my comment on the diameter being important stands. Also important will be the mass of the coating supported by the weakest point, usually the coating/substrate interface.

Hi gordon,

Bearing seat diameter is 40 mm and length (width) is 30 mm. Part RPM is 12000.

My question is actually exactly what Djewell pointed out; how do we know when the centrifugal force of a rotating part (that is at what RPM) exceeds the bond strength of the coating / cohesive strength of the particles resulting in coating failure?

Regards

Shantanu
11-19-2013, 09:12 AM, (This post was last modified: 11-19-2013, 09:24 AM by kschewe.)
 kschewe Senior Member Posts: 94 Threads: 1 Joined: Oct 2010 Reputation: 3
RE: Maximum allowed RPM
I guess it would depend on bond strength vs centrifugal forces.
I know we coat area of the impeller on a helicopter engine and it rotates between 30k-50k crazy fast.
similar dimensions to yours but much faster rpm. oem book repairs...

doing rough calculations probably less then 2000 psi of force on the coating. Newton's laws
11-19-2013, 12:00 PM,
RE: Maximum allowed RPM
Hi Shantanu

For bearing seat diameter is 40 mm and length (width) 30 mm. Part RPM 12000, with a coating thickness of 1mm the stress generated at interface will be very roughly in this sort of order:

Tungsten carbide based coating: 0.5 MPa
Steel based coating: 0.25 MPa

So this should be well within working limits of most coatings.
11-22-2013, 07:01 PM,
 shantanu Senior Member Posts: 88 Threads: 32 Joined: May 2011 Reputation: 1
RE: Maximum allowed RPM
(11-19-2013, 12:00 PM)Gordon Wrote: Hi Shantanu

For bearing seat diameter is 40 mm and length (width) 30 mm. Part RPM 12000, with a coating thickness of 1mm the stress generated at interface will be very roughly in this sort of order:

Tungsten carbide based coating: 0.5 MPa
Steel based coating: 0.25 MPa

So this should be well within working limits of most coatings.

Hi Gordon,

Numbers always help!

Thanks so much

Shantanu
11-29-2013, 04:30 PM, (This post was last modified: 11-29-2013, 04:30 PM by Vijay Deshpande.)
 Vijay Deshpande SuperMember Posts: 131 Threads: 44 Joined: Dec 2006 Reputation: 2
RE: Maximum allowed RPM
Dear Shantanu,
Further to all posts, I would like to add that we have succesfully sprayed turbocharger shafts running at 36000 RPM with Arc spray.
Vijay

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