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A pressure loss that persists through a fresh seal replacement is one of four things: the bore that the new seal runs against is worn beyond the seal's compensation range, a different circuit position not included in the kit is bypassing, the accumulator pre-charge has dropped and is causing pressure cycling that reads as a loss, or the new seal was installed with an error that became apparent only under operating load. Eliminating these one at a time with targeted tests takes 40–60 minutes and avoids the guess-work teardown that costs 8–12 hours.
The bore measurement is the first check — before the second teardown. A percussion bore worn to 0.20 mm above OEM diameter cannot be sealed by any elastomer lip seal regardless of Shore hardness or compound. The contact stress at the bore falls below the minimum required to maintain oil film exclusion. This is the most common explanation for post-installation pressure loss in drills above 6,000 total operating hours. The solution is cylinder body replacement, not a second kit change. The distinction saves 4–6 hours of teardown labor and one kit cost at the price of a 15-minute bore measurement.
Persistent Pressure Loss Diagnostic After Seal Replacement
|
Possible Cause |
Diagnostic Test |
Result Indicating This Cause |
Required Action |
|
Percussion bore worn beyond seal compensation range |
Bore gauge measurement at 3 axial positions and 2 angles |
Bore diameter more than 0.15 mm above OEM spec (0.10 mm for gray iron) |
Cylinder body replacement — new seal cannot hold on worn bore |
|
Accumulator pre-charge below specification |
N₂ pre-charge gauge at accumulator charge port |
Pre-charge below 10 bar of specification (e.g., 45 bar where 65 bar is spec) |
Recharge to specification; if won't hold, replace diaphragm |
|
Circuit position outside the kit still bypassing |
Isolate percussion circuit; test rotation and feed circuits separately |
Pressure holds on percussion; drops on rotation or feed isolation test |
Replace seals in the identified bypass circuit |
|
Installation error — reversed or under-lubricated seal |
Pressure-hold test at 140 bar with percussion blocked for 90 seconds |
Drop of more than 6 bar in 90 seconds at static hold confirms bypass |
Disassemble; inspect seal orientation and lubrication; reinstall correctly |
|
Oil contamination above ISO 17/15/12 attacking new seal |
Pull oil sample for particle count within first 50 hours after installation |
ISO cleanliness above 18/16/13 — particle count elevated |
Flush circuit to ISO 16/14/11; replace filter elements; reseal |
The 90-second static pressure hold test done immediately after installation and before the drill returns to service catches installation errors at the bench rather than at the face. 90 seconds of testing versus 8 hours of teardown — this is the most underutilized check in percussion seal maintenance. HOVOO provides post-installation pressure verification procedures with all full-kit seal packages for Atlas Copco and Sandvik drifters. References at hovooseal.com.
