A hydraulic motor shaft seal is a radial rotary lip seal engineered to retain hydraulic case drain fluid and prevent external atmospheric contamination around the drive shaft.
Replacing a shaft seal on an axial piston motor (such as the Rexroth A2FM, A6VM, or equivalent Huade A2F series) requires verifying case drain pressure, inspecting the shaft surface for wear grooves, and seating the seal to precise mechanical tolerances.
When installed with proper lip protection and correct orientation, a high-quality FKM or NBR radial shaft seal prevents hydraulic fluid leakage and maintains motor volumetric efficiency.
Why Do Axial Piston Motor Shaft Seals Leak?
A premature shaft seal failure is usually a secondary symptom of an internal hydraulic circuit malfunction rather than a defective seal lip.
Before replacing the seal, technicians must diagnose the underlying root cause to prevent recurring failure:
- Excessive Case Pressure: Axial piston motors rely on internal leakage fluid to lubricate bearings and rotating groups before returning to the reservoir via the case drain line. If the case drain line is restricted or the tank filter is clogged, internal pressure exceeding 2.5 to 3 bar (36 to 43 psi) will blow the seal lip off the shaft.
- Excessive Internal Wear: Worn valve plates, loose piston shoes, or barrel lift increase internal bypass flow. When leakage exceeds the case drain line's discharge capacity, internal pressure spikes rapidly.
- Shaft Runout and Bearing Play: Excessive radial or axial play caused by worn tapered roller bearings allows the drive shaft to deflect eccentrically, breaking continuous lip contact.
- Thermal Hardening: Operating temperatures above fluid ratings cause nitrile (NBR) rubber to vulcanize, crack, and lose sealing elasticity.
Diagnostic Check: The Motor Case Drain Test
Before removing the motor from the machine, perform a case drain flow measurement to verify internal rotating group health:
- Disconnect the Case Drain Line: Disconnect the drain line (Port T or L) from the motor housing and route it into a clean measurement container.
- Operate Under Load: Run the hydraulic system under normal working pressure and operating temperature for 60 seconds.
- Measure Leakage Volume: If case drain leakage exceeds 5% to 10% of the motor's rated maximum input flow, the internal rotating group is severely worn. Installing a new shaft seal without rebuilding the rotating group will result in immediate re-leakage.
Required Tooling and Preparation Specifications
Disassembling and installing radial shaft seals requires dedicated mechanical tools to prevent damaging the motor housing bore:
Step-by-Step Shaft Seal Replacement Procedure
Follow these five mechanical steps to extract the failed component and press-fit the new seal:
Step 1: Housing Decontamination and Port Sealing
Clean the external motor housing thoroughly with solvent degreaser. Plug all hydraulic ports (A, B, and case drain T) with threaded steel plugs. Preventing solid particulate ingress prevents abrasive wear on internal swashplates and cylinder blocks.
Step 2: Retaining Ring Removal
Using precision internal snap-ring pliers, compress and extract the external retaining circlip. Inspect the circlip for axial bowing. A bowed circlip indicates the motor experienced severe case pressure shockwaves.
Step 3: Seal Extraction (The Threaded Pull Method)
Avoid prying between the shaft and housing with screwdrivers. Instead:
- Drill two small pilot holes (1.5 mm diameter) into the exposed metal casing of the seal, positioned 180° apart.
- Thread two self-tapping sheet metal screws into the pilot holes.
- Apply equal tension using slide hammers or locking pliers on both screws to pull the seal straight out of the housing bore.
Step 4: Shaft Surface Evaluation and Tolerance Check
Inspect the shaft sealing track where the elastomer lip was running:
- Visual Inspection: Clean the sealing diameter. Check for circumferential grooves, corrosion pitting, or scoring.
- Tactile Check: Run a fingernail axially across the contact path. If your fingernail catches in the groove (depth > 0.05 mm), a replacement seal will leak within 100 operating hours.
- Groove Remediation: If grooving is detected, install a precision stainless steel wear sleeve (Speedi-Sleeve) over the damaged section, or replace the drive shaft.
Step 5: Press-Fitting the New Radial Seal
- Grease Packing: Pack the void between the primary sealing lip and the secondary dust wiper lip with high-temperature NLGI 2 grease. This grease barrier prevents dry-friction burning before hydraulic oil reaches the shaft interface.
- Spline Protection: Slide an installation sleeve (bullet) or wrap zero-residue PTFE tape smoothly over the drive shaft splines and keyway.
- Orientation: Align the seal with the garter spring facing inward toward the hydraulic fluid (fluid pressure must energize the sealing lip).
- Axial Pressing: Place the seal driver squarely against the outer metal casing. Press or gently tap the driver until the seal seats squarely against the internal housing shoulder. Do not apply force to the elastomer center.
- Reinstall Circlip: Reinstall the retaining ring into its machined groove, verifying 360° engagement.
Seal Material Selection: NBR vs. FKM (Viton)
Selecting the correct elastomer compound prevents chemical degradation and thermal hardening:
*Recommendation: For heavy-duty industrial machinery operating continuously under high ambient temperatures, specify FKM seals. For mobile equipment operating in Arctic or sub-zero environments, specify NBR seals.*
Pre-Startup Verification: Motor Case Priming Protocol
Never start an axial piston motor dry after seal installation.
Before connecting the main system lines, fill the motor housing completely with clean hydraulic oil through the highest case drain port.
Starting an unprimed motor causes immediate frictional burning on the dry shaft seal lip and catastrophic seizure of the cylinder block cradle bearings before fluid can circulate.
High-Quality Axial Piston Motor Sourcing
If your motor exhibits excessive case drain flow, shaft grooving, or bearing failure, replacing the shaft seal will not restore operational efficiency.
To explore dimensionally interchangeable drop-in replacement units for Rexroth A2FM, A6VM, and A2FE series, download the complete Huade Axial Piston Motors Technical Catalog.
Our engineering team provides direct cross-reference verification and seal kit replacement specifications. Submit a Technical Inquiry to verify your hydraulic motor dimensions today.