Introduction / Process Question

[Paul9912]

I have recently been involved with a work related project that involves revising an existing process which has shown to be an unsuccessful “fix” for the component after it has been in service. In short, the process involves inserting a cylindrical slug into a larger body that has had a hole EDM drilled out. The slug essentially replaces the material removed during EDM and is held in place with an interference fit of approximately .0005 - .001 and also with the use of Loctite 620 retaining compound. Prior to inserting using an arbor press, the bore is checked with Go/NoGo class z pins to determine if EDM has kept their ID tolerances (+/- .001). The slug is manufactured with an OD Tolerance of +/- .0005.

The issue we see is that these slugs are coming loose while the component is in service causing the customer unexpected downtime. The conditions surrounding the component include an operating temp of 200-300 F and Hydraulic pressures / pulsations of approximately 450 bar.

Being a community of professionals, I’m sure you may already have had a “red flag” thrown and blanks drawn with my description, I do apologize. I would like to list my thoughts on how to improve the process and get your input on each; also, any suggestions of other areas I should look into that I am not touching on would be appreciated.

Please let me know if I can help clarify any info.

1. Have body bore EDM drilled slightly undersized and then finish to size using jig grinding. This will improve surface finish of bore while removing slag layer created during EDM. Jig grinding should also be a more cost effective method for maintaining a tolerance of +/- .0005 as compared to EDM.

2. Inspect bore after jig grinding with (+)(-) z class pins. If work is questionable, then inspect using same size x class pins.

3. Have slugs manufactured from material that is more comparable to the body material. The body is manufactured with 4140 alloy steel. The slugs are currently manufactured using D2. Both are tempered to a Rockwell value of 54 – 60 HRc.

4. Increase interference and freeze fit slug into body in order to reduce the chance of damage to the ID bore during inserting. The use of Loctite 620 would still be used to aid in retaining / sealing the fit.

 

[tygerdawg]

Can the plug be installed PERMANENTLY with no anticipated subsequent removal?
Must it be a fluid seal at hydraulic pressures?
What is finish requirements of part surface?

My first thought would be to laser weld the slug in place. To offset the equipment cost, the hole & slug tolerances can be increased and process time decreased.
Then I thought about a mechanical rollover process on the slug ends.
Then I thought about a simple material disruption clinching process to hold the slug in place.

I'm sure there's other smart folks out there who've had more coffee than I have and could give additional/smarter ideas.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[Jboggs]

Years ago I did my first work with complex industrial hydraulic systems and discovered a type of plug that seems ideal for your application. It consists of small cylindrical barrel with a steel ball inserted in one end. You have to machine the hole with a small shoulder to support the fitting, but accuracy, precision, and fit are not critical. You simply drop the fitting into the hole and drive the ball home with a hammer. As the ball is driven into the barrel it expands the sides and permanently seals the port. We never had any leaks. The only way to get these things out is to drill them out, and that is easier said than done. They are available from several sources. Any experienced industrial hydraulic shop should be familiar with them. Here are some manufacturers:

http://www.eisinserts.com/install.html

http://www.kvt-koenig.us/Products/

 

[Oldhydroman]

Hi Paul

Following Jboggs' theme, you might also want to check out "Lee Plugs" (see link below). They have several ranges of plugs which might do the trick. As well as the typical Koenig/CV type plug, Lee have special designs for "axial force only" and for "radial force only".

http://www.theleeco.com/PLUGWEB2.NSF/Plugs!OpenView

Also check out the tolerancing requirements of the standard CV type plug - it is often the case that a simple drilled hole is sufficient. Some types even prefer the surface finish you get from a drill rather than a reamer.

For subsequent removal (if necessary) you can drill out the plugs with a standard shop drill, then tap a hole, fit a screw/bolt and pull it out (you might need a slide hammer affair). Sometimes you can then fit an "oversized" plug to go into the "already used once" hole. Or else you drill it out the next size up (remembering that both metric and inch sizes are available) and then start all over again.

DOL

 

[GregLocock]

I have a strong feeling that Loctite is for use in clearance holes, not interference. Anyway bashing a hard ball in sounds much better to me.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Paul9912]

Hello All,

Thank you for your responses and ideas. I believe it is necessary for me to go into more detail in order to give you a better idea of what I am working with.

The insert or slug is a replacement valve seat for a one way hydraulic check valve. This valve operates at a high rate and over time (11,000 - 14,000 hrs.) wears the original integral seat to a point that the sealing and operation characteristics of the valve is no longer acceptable. At this point the integral seat is EDM'ed and the new inserted seat is installed, allowing the valve to operate as it did new.

I have attached a rough drawing of the components involved for your reference.

In response to tygerdawg:
[ul]
[li]The EDM services and manufacture of the inserts are outsourced to local companies. We see about 300 units annually so tooling up would not be profitable.[/li]
[/ul]
[ul]
[li]Yes, we would like the insert to be a permanent fit.[/li]
[/ul]
[ul]
[li]The fit must be a hydraulic seal.[/li]
[/ul]
[ul]
[li]As of now there has not been any surface finish requirements for EDM or the insert. FYI the OD of the insert is ground to size.[/li]
[/ul]

I hope this info is helpful. Any thoughts on the process improvements I suggested? I'm curious to know if I'm on the right track with these ideas.

 

[desertfox]

Just a couple of observations on the sketch you've provided:-
There appears to be no lead in chamfer on slug/body to help slug/body assembly, also if the two parts are supposed to be bonded using loctite then it might be better to provide some grooves in the slug for the loctite to sit in while its assembled, otherwise the interference fit as you assemble the parts scrapes the loctite off as the slug is inserted.
Finally whilst increasing the interference between the two components may well prevent it coming loose, as anybody calculated the loads on the slug/body at the elevated temperature to see whether the original interference is adequate?
Bear in mind also, even if the slug/body have the same coefficient of expansion unless they are uniformly heated then
the fit can still be lost through differential heating.

desertfox

 

[hydromarine]

Cant see much wrong with the process for the seat insertion, but if the seats are not holding with the clearances you have given plus loctite I would consider electron beam or laser welding and compatible materials for valve seat and valve ie stellite, had this same problem with loose seats on a high pressure water jetting machine some years ago , the unloading valve seat set at 15,000 psi
was working loose on a regular basis

 

[Oldhydroman]

Hi Paul

Get hold of a copy of the Bosch Rexroth datasheet RE20380 and look at the way the M-SR*KD valve seats are installed. This design uses a shrink fit seat (-60 deg C) and seems to work for them. There may be some pointers for you in their design.

Your working pressure is higher than the rating of the standard Rexroth valve but you might find that they have a special high pressure version. If so, there may be a possibility that your rework could be to replace the original assembly with the M-SR*KD type insert. (I don't work for them so it's not a sales pitch I'm making here.)

Also take a look at the "Lee chek" valve. They have a 8,000 psi version which is installed in a simple reamed hole using the same type of expanding mandrel as the plugs discussed earlier. They also have a 15,000 psi, NACE compliant version for downhole tools for the oil and gas industry. That is an extremely harsh environment so if the valve can tolerate that it might be OK for you.

DOL