DBA Forum (B) - Chris Roy
2014-08-10 14:19:44 UTC
With some trepidation, I recently repacked my stuffing box. See attached diagram for an overview. [url=Loading Image...
&imgrefurl=http%3A%2F%2Fenginemechanics.tpub.com%2F14075%2Fcss%2F14075_127.htm&h=348&w=607&tbnid=HLZ4Jh_8IydZSM%3A&zoom=1&docid=5QMvezZYJyPaAM&ei=dn7nU56TL6eP0AW2_4HACw&tbm=isch&client=firefox-a&iact=rc&uact=3&dur=2689&page=1&start=0&ndsp=15&ved=0CC4QrQMwBA]Here[/url]
Not as difficult as I originally believed and Claes Compaen is still above the water, so I thought I would share my experience in the hope it will help others, in the same boat so to speak.
Standard packing is available either as PTFE or graphite (the cheapest) impregnated.
Apparently PTFE gland packing is for specialist use in the food and pharmaceutical industry. It is not really suitable for high speed rotating shafts which handle water that contain sediment such as grit and clay as is the case of prop shafts.
One member reports that he “once had the pleasure of repacking a stern tube that had been stuffed with PTFE, where due to over tightening the temperature had risen and the PTFE melted. The molten PTFE on cooling even formed a plug in the grease hole stopping further lubrication.
It was a devil of a job getting it out of the stuffing box and replacing it with traditional graphite impregnated cotton packing”.
Nuff said, so I chose the graphite.
Thickness is important to ensure a reasonably tight fit.
I measured the thickness of the packing gland (the part that compresses the packing around the prop shaft) at 15mm and found that ASAP only supplied 13 or 16mm.
Forum advice was to get the 16mm and hammer it thinner on a clean flat surface.
Alternatively I could have bought the 13mm and hammered it thicker, but as this stuff does seem to regain its shape after a short while, I went for the 16mm.
I also bought a special and vital tool for removing the old packing. This is a long flexible shaft with a T bar on one end and a sharpened coil of strong wire on the other. It acts like a flexible corkscrew.
Down to business.
I set up an emergency pump, just in case the worst happened, injected a load of grease into the stern tube to seal out the canal water and removed the packing gland. Using the special tool I then pulled out all the old pieces of packing being careful not to scratch the shaft.
At least 3 individual packing rings must be inserted, each butting together as seamlessly as possible. My first attempt to cut the first ring to size was a complete failure. I used a length of string to measure the circumference of the shaft and then cut (90 deg.) a corresponding length of packing.
It was short by several millimetres. A simple mistake because when the packing is wrapped around the shaft it contracts due to the thickness and construction of the material.
On my second attempt I wrapped the packing around the shaft and overlapped the join, before using a new Stanley knife blade to make one cut across both ends at a 45 degree angle to the vertical.
Bashed it again with the hammer and slid it into the stuffing box, noting that the join was virtually seamless and also, most importantly, where on the shaft the join was located.
I used the packing gland to push the ring right into the box, before pulling the gland out ready for the next ring.
The next 2 rings were a repeat of the first, except that I was careful to offset the joins by around 120 deg. to avoid leaving a straight run of joins and therefore easy passage for the ingress of water.
Finally, I ever so slightly tightened up the packing gland to ensure the packing stayed in position but was not compressed.
Once the surfeit of grease in the shaft has dispersed, I will tighten the gland appropriately to ensure only one or two drops of water per minute come through when under way, easily coped with by the nappy I leave under the box just for that purpose.
With hindsight, simples!
&imgrefurl=http%3A%2F%2Fenginemechanics.tpub.com%2F14075%2Fcss%2F14075_127.htm&h=348&w=607&tbnid=HLZ4Jh_8IydZSM%3A&zoom=1&docid=5QMvezZYJyPaAM&ei=dn7nU56TL6eP0AW2_4HACw&tbm=isch&client=firefox-a&iact=rc&uact=3&dur=2689&page=1&start=0&ndsp=15&ved=0CC4QrQMwBA]Here[/url]Not as difficult as I originally believed and Claes Compaen is still above the water, so I thought I would share my experience in the hope it will help others, in the same boat so to speak.
Standard packing is available either as PTFE or graphite (the cheapest) impregnated.
Apparently PTFE gland packing is for specialist use in the food and pharmaceutical industry. It is not really suitable for high speed rotating shafts which handle water that contain sediment such as grit and clay as is the case of prop shafts.
One member reports that he “once had the pleasure of repacking a stern tube that had been stuffed with PTFE, where due to over tightening the temperature had risen and the PTFE melted. The molten PTFE on cooling even formed a plug in the grease hole stopping further lubrication.
It was a devil of a job getting it out of the stuffing box and replacing it with traditional graphite impregnated cotton packing”.
Nuff said, so I chose the graphite.
Thickness is important to ensure a reasonably tight fit.
I measured the thickness of the packing gland (the part that compresses the packing around the prop shaft) at 15mm and found that ASAP only supplied 13 or 16mm.
Forum advice was to get the 16mm and hammer it thinner on a clean flat surface.
Alternatively I could have bought the 13mm and hammered it thicker, but as this stuff does seem to regain its shape after a short while, I went for the 16mm.
I also bought a special and vital tool for removing the old packing. This is a long flexible shaft with a T bar on one end and a sharpened coil of strong wire on the other. It acts like a flexible corkscrew.
Down to business.
I set up an emergency pump, just in case the worst happened, injected a load of grease into the stern tube to seal out the canal water and removed the packing gland. Using the special tool I then pulled out all the old pieces of packing being careful not to scratch the shaft.
At least 3 individual packing rings must be inserted, each butting together as seamlessly as possible. My first attempt to cut the first ring to size was a complete failure. I used a length of string to measure the circumference of the shaft and then cut (90 deg.) a corresponding length of packing.
It was short by several millimetres. A simple mistake because when the packing is wrapped around the shaft it contracts due to the thickness and construction of the material.
On my second attempt I wrapped the packing around the shaft and overlapped the join, before using a new Stanley knife blade to make one cut across both ends at a 45 degree angle to the vertical.
Bashed it again with the hammer and slid it into the stuffing box, noting that the join was virtually seamless and also, most importantly, where on the shaft the join was located.
I used the packing gland to push the ring right into the box, before pulling the gland out ready for the next ring.
The next 2 rings were a repeat of the first, except that I was careful to offset the joins by around 120 deg. to avoid leaving a straight run of joins and therefore easy passage for the ingress of water.
Finally, I ever so slightly tightened up the packing gland to ensure the packing stayed in position but was not compressed.
Once the surfeit of grease in the shaft has dispersed, I will tighten the gland appropriately to ensure only one or two drops of water per minute come through when under way, easily coped with by the nappy I leave under the box just for that purpose.
With hindsight, simples!