Sealing a shaft for concrete delivery screw 2

[Grindy]

I'm looking for some advice/ideas about how I can seal a drive shaft that turns a concrete delivery screw. At the moment the drive shaft is sealed by a gland packing material called Duramid (manufacturer James Walker). The drive shaft passes through a replaceable steel ring which fits through the wall of the screw housing. There are then three rings of Duramid gland packing which butt up against this steel ring. A compression ring then pushes up against the gland packing to ensure a tight seal around the shaft.

The shaft runs at about 60 RPM. The diameter is 60mm. The drive shaft material is mild steel with a nitride coating. The shaft runs intermittently and is constantly exposed to concrete during the working shift.

The current problem is, over time the concrete (grit particles) get between the gland packing and the drive shaft and destroys both the shaft and packing. The gland packing is only lubricated during installation and cannot be lubricated once in service.

Any help or advice would be appreciated.

For reference I'm based in the UK

 

[MiketheEngineer]

As you have found - concrete is "NASTY" stuff. If you are making this - you will need to do a bunch of research. If you bought it - call the mfg and get them to figure a solution.

 

[btrueblood]

In other abrasive slurry pumps, the seal might be lubricated/rinsed/cooled by a clear barrier fluid, perhaps one that is filtered and recycled. A secondary barrier could reduce but probably not prevent, the barrier fluid from mixing with the pumped liquid. Probably not do-able with concrete, without detriment to the concrete strength or cure time, or some other parameter. Even air as a barrier fluid would tend to put bubbles in the concrete, which from memory is a big pain to remove from concrete once poured. About the best you could do is find a sealing system that lent itself to simple, rapid replacement, preferrably without disturbing any critical alignments...

 

[MintJulep]

Might be one of those cases where you admit that it's just about impossible to stop the parts from wearing out and just make things cheap and easy to repair.

Increase the size of the existing replaceable ring and packing gland.

Add a replaceable split collar over the shaft where it passes through the gland.

 

[berkshire]

Is this part of a bin discharger, or can the end of the screw be located away from the concrete input?
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[DAVIDSTECKER]

For my first line of defence I would use an SKF VR series V-Ring on the concrete side of the packing gland (Stuffing box). This rotating face seal keeps moving particals outwards away from the shaft and packing gland area, They are pretty cheap too.
Then drill and tap for a water jubrication feed into the packing area, modify gland the screws into place or is held in place with studs to accept a steel cartage lip seal.
Once the gland side can hold the water in you can pump a small amount water into the packing rings to lubricate them and cause a positive pressure flow to keep the abrasive particals flowing out. Check out Johnsons Duramax Marine Heavy duty stuffing boxes for different design ideas that might work for your current hardware.
Being in the UK I would expect that you find a friendly shipyard to do some research at.
Agian my first step would be to install the V-ring since they are used offten on the outboard shaft bearing housings of off road trucks and farm equipment to keep the dirt, grit, and tall grass away from the normal lip seals and bearings.
In my old boat, the packing gland was correctly adjusted when I could count about 6 to 10 drops of water a minute coming out of the gland when the shaft was turning. The was required for proper lubrication of the packing. I do not think that small amount of water would effect your concrete.
I also used Garloc (Gortex) PTFE packing with septacular results.
Find your local rep and ask for an engineering sample of about 10 feet of the size you need.
Keep us posted on your findings.
Best regards, David (Schoellhorn-Albrecht Mach. Co. USA)

 

[micalbrch]

Install a stainless steel shaft bushing between shaft and packing gland. Then you must only renew the bushings and the packing but not the whole shaft when the seal area wears out. I saw this design not on screws for concrete but for waster water cake which contained a lot of sand.

 

[1gibson]

Packing doesn't work out very well without lubrication.

Whoever told you that no lubrication (not even a few drops of water per minute!?!) is allowed during use must be in charge of the concrete mix quality? I'd ask to see their measuring equiment, and unless they can account for water-cement ratio down to 5 decimal places, I'd ask them to kindly reconsider that requirement.

 

[Grindy]

Thank you all for the information so far. It has given me plenty to think about.

F.A.O. 1Gibson. No one has said we can't have lubrication it is just that the current design/setup does not allow us to get lubrication between the shaft and the gland packing once the screw is in service. I can pump grease into the stuffing box, but due to the fact the gland packing is so tightly compressed the grease cannot pass between it to reach the shaft. The grease simply goes for the path of least resistance which is between the stuffing box and the outer surface of the gland packing, resulting in grease sliding out past the compression ring.

 

[DAVIDSTECKER]

Grindy,
If you already have a grease fitting you could put one of those wavy washers in between the packing rings in line with the grease fitting to create a passage. Or leave a gap in one of the rings and align it with the fitting during assembly.
Secondly you can over tighen the packing which can damage your even without the grit of the cement.
As I mentioned before, it needs to be loose enough for water to drip out.
Once you have the grease passage created and the packing loosened up, you can install a spring powered grease injector or Lincon has battery powered ones, I think some even have timers, that should cause a positive flow keeping the cement from getting in to the works.

 

[Hidromar]

Hi!

We've got a case like this one.

Being such a low speed, we proposed to make a self-energising seal on plastic. To ensure concrete if "off", you can use a brass scrapper to prevent concrete affects the seal

We are based in Madrid. In any case, please, let me know or get a drawing about jow the housing is actually.

http://www.hidromar.es

 

[MikeHalloran]

I'm wondering if Caterpillar's Duo-Cone metal face seals could be adapted.

http://www.cat.com/cda/components/fullArticle?m=111102&x=7&id=1142540

Mike Halloran
Pembroke Pines, FL, USA

 

[Hidromar]

Interesting point, MikeHalloran

This kind of seals are widely used on machinery's wheel's axes in order to avoid inlet of mud, sand...

This one can be considered also!

http://www.hidromar.es

 

[2Thou]

Hi Grindy
You could do a lot worse than speak to SW Plastics in Sevenoaks about a packing material specifically developed for your application within the concrete batch industry. Cheap and effective.

 

[Cockroach]

Run a wiper ring to the outside of the packing and grease inject into the packing, hold internal pressure thereof using a check valve at the injection point.

Works well in drilling, sand applications incurred during application. Gotta be roughly the same, except yours is much less dynamically affected.

Regards,
Cockroach

 

[Grindy]

Thank you all for your advice so far. I had to carry out a drive shaft replacement the other day so we decided to trial some of the ideas you have all offered so far.

Currently we still have the same stuffing box and stainless steel ring fitted into the screw housing. However we now have a single lip seal with auxiliary lip fitted after the stainless steel ring, then a steel ring with holes running through it to allow grease to pass through it to lubricate the seal and the packing. There are now only two pieces of packing (due to the addition of the seal and steel ring). The packing is then very lightly compressed by the compression ring.

The stuffing box is being greased through the nipple in the stuffing box daily. An auto greaser is going to be fitted shortly.

The reason for the installation of the single lip seal was due to the fact we use them on our cement delivery screws and they have proved to be successful in that application. Also we had them to hand!

The fitting of a V ring seal (as recommended by David Stecker) is also being looked into.

There is some discussion about which way the single lip seal (with auxiliary lip) should be installed. Should the primary lip be fitted toward the stuffing box or toward the concrete? Can I also have your reasoning for why you would install the primary seal one way or the other?

Thanks again

 

[Hidromar]

Grindy

May you share a drawing?

http://www.hidromar.es

 

[Grindy]

I have added a screen print of the basic design. The seal (in red) only shows the primary lip.

 

[Grindy]

Update

Hello all, just a quick update as to my shaft sealing problem. Currently we are using standard single lip radial shaft seals, fitted in the correct orientation so as to prevent the ingress of dirt. Seal size 90 x 60 x 10

The stuffing box is unchanged and after the first seal we have added a steel ring, complete with holes to allow grease to reach the shaft. Two seals are then fitted after the ring to act as back ups.

Currently this setup is proving successful, 10+ weeks without any seal damage or leakage. The seals are only greased every 3 weeks with a general purpose grease.

The only other addition has been the fitting of an abrasion resistant plastic disc which sits within the wet hopper between the wall and the horizontal screw end plate. This reduces the gap and prevents excessive ingress of concrete. This was fitted prior to checking the seals after 6 weeks of successful operation.

Thanks again for all your advice