Vicsec VC13 mini-excavator

   / Vicsec VC13 mini-excavator
  • Thread Starter
#51  
After more research, I believe that the pump outlet port fitting is DIN 3852-H (adjustable o-ring style); it has a locking nut, backup washer, and retaining ring. From that, I think a DIN 3869 profile ring (aka "D-rings") might be the best solution, particularly since the backup washer does not fit snugly on the fitting shank. The two o-rings that I installed have both been extruded through that gap between the backup washer and the fitting. The D-Ring profile has a flat top and outer radius; those surfaces would seat flush against the backup washer and retaining ring, respectively.

Assuming that analysis is correct, it looks like there are plenty of suppliers that provide these in Viton/FKM. But to pick the right one, I need to tear the machine down again and measure the fitting's OD and the washer's ID and thickness. With luck, I can find a local hydraulic parts supplier with the one that I need in stock, so I don't have another weekend or two of machine downtime.

If I can't find a suitable D-ring locally, I will try to find a standard FKM o-ring with 90A hardness. If I can't find either, I might try to limp along with another FKM o-ring (probably 70A) from the kit that I have on hand. Otherwise, I will be stuck waiting for suitable parts to be delivered.
 
   / Vicsec VC13 mini-excavator #52  
After more research, I believe that the pump outlet port fitting is DIN 3852-H (adjustable o-ring style); it has a locking nut, backup washer, and retaining ring. From that, I think a DIN 3869 profile ring (aka "D-rings") might be the best solution, particularly since the backup washer does not fit snugly on the fitting shank.
Just looking at the spare metric O-ring fittings I have, and the DHH catalog, it seems the fittings that need to be "clocked" have to have a backup washer in addition to the retaining ring. So, in comparing a straight fitting to a 90-degree fitting (e.g. 6068 vs. 6069), the straight one has no backup washer.

All my spare fittings have a snug 0-ring.
 
   / Vicsec VC13 mini-excavator
  • Thread Starter
#53  
I just went out and pulled the fitting to measure the seal dimensions. This time, I brought my camera and took a photo of the failed ring in the fitting. You can see from the picture of that the O-ring was pushed between the gap between the backing washer and fitting. Again.

Based on some quick measurements with my calipers, the fitting OD is ~18.12mm, the retaining ring ID is ~23.6mm, and the retaining ring thickness is ~1.75mm. Based on those measurements, I’m thinking an 18.5mm ID o-ring with 2.0 (or 2.4?) mm cross-section will compress properly and fill the space.
 

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   / Vicsec VC13 mini-excavator
  • Thread Starter
#54  
Just looking at the spare metric O-ring fittings I have, and the DHH catalog, it seems the fittings that need to be "clocked" have to have a backup washer in addition to the retaining ring. So, in comparing a straight fitting to a 90-degree fitting (e.g. 6068 vs. 6069), the straight one has no backup washer.
I just checked discount hydraulic hose, and it looks like this fitting is a 5069L? Viewing the various sizes available made me realize that I forgot to measure the thread pitch while I had it off the machine. Guessing it is M22x1.5?

I also had the thought that this failure might relate to engine speed? Specifically, is pressure proportional to RPMs? Could I be causing an over-pressure condition by operating the machine with the throttle wide open? Maybe the upper limit needs to be adjusted downward? I still need to do a pressure test and adjust the main RV, so this line of thinking might bump that up on my priority list.
 
   / Vicsec VC13 mini-excavator #55  
I also had the thought that this failure might relate to engine speed? Specifically, is pressure proportional to RPMs?
No.

But everyone should do a pressure test when they first get their machine and set the main RV to 2400-2500psi. Most are set way too low.
 
   / Vicsec VC13 mini-excavator
  • Thread Starter
#56  
I have ordered some 90A FKM o-rings that I hope will be less prone to extrusion failure.

In the meantime, I realized that there might be a trick to installing this fitting in a way that reduces the chance of this type of failure. Since this side of the fitting installs into the pump horizontally, the backup and retaining washers will be pulled downward by gravity, so they rest on the top of the fitting. That means there will be zero gap at the top of the backup washer and double the average gap at the bottom. That’s probably where the extrusion happens. I need to be sure that the washer is centered (so the gap is even all around) before tightening down the nut.
 
   / Vicsec VC13 mini-excavator #57  
   / Vicsec VC13 mini-excavator
  • Thread Starter
#58  
I continue to have leaking issues at the pump output, so this weekend I took a deep dive into the world of o-rings. If you find yourself in this situation, there are a couple of resources worth investigating:
  • Parker O-Ring Handbook: A comprehensive guide to o-ring materials, applications, static and dynamic sealing designs, back-up rings, and material compatibility. Something like this should be required reading when engineering with o-rings.
  • Apple Rubber Gland Calculator: Interactive tool for determining the proper design of o-ring glands (the groove/channel in which the o-ring seats). I have been using this tool to check the fit of different o-ring sizes in the gland created by the stack up of my fitting's backup washer and retaining ring.
Based on these tools, here are some quick notes relevant to my current situation:
  1. In this application, the o-ring is being compressed axially (top to bottom) in a static configuration to seal internal pressure. The compression "squeeze" should be between 10-40% of the cross section (CS). Too little, and it won't seal properly; too much and it will fail prematurely via deformation or extrusion.
  2. The ID of the o-ring should be smaller than the OD of the fitting shaft, such that the o-ring seals tightly; however. the difference should be less than 5%.
  3. The elastomer materials generally are considered incompressible, so the volume of the o-ring remains nearly constant when squeezed. In this case, axial compression results in radial expansion. Meanwhile, the volume of the o-ring should not exceed 90% of the volume of the gland. For uniform o-rings and glands, this ratio can be calculated by looking at the cross sectional area. The CS area of the o-ring is a circle (A_ring = ½ × CS × π²), and the CS area of the gland is a rectangle ( A_gland = (OD - ID) × H). Thus, the ratio of cross sectional areas should be A_ring / A_gland < 90%.
As stated in a previous message, my fitting gland is defined by its 18.1mm shaft and the retaining ring (23.6mm ID x 1.75mm H), creating a gland groove that is 2.75mm W x 1.75mm H. By applying the above rules, the following o-ring requirements emerge:
  1. The CS must be between 2.2mm and 2.4mm.
  2. For my ~18.1mm fitting shaft, I should find an o-ring with an ID between 17.2mm and 18mm.
  3. The Apple Rubber tool calculates the volume ratio automatically when given the other parameters.
After playing with the Apple Rubber tool, I cannot find any existing o-rings that meet all of these constraints. If the CS is too small, there's not enough squeeze; if it's too big, the ring has too much volume to fit properly in the gland. The FKM o-rings that I have been using from HF measures 17.5mm x 2.56mm; these produce both too much squeeze (32%) and too much volume (105%), so I'm not surprised that I have been seeing extrusion failures.

That said, the diagram on the tool page contradicts constraint #2; it shows the o-ring being pushed outward (into the retaining ring). This leaves a gap along the shaft, which would allow the fluid to leak past the backup washer. So I'm not sure of anything at this point.
 
   / Vicsec VC13 mini-excavator #59  
  1. The ID of the o-ring should be smaller than the OD of the fitting shaft, such that the o-ring seals tightly; however. the difference should be less than 5%.
Are you ready to try an all-new fitting yet?
 

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