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Ashcroft Locker

 

Ashcroft Air Locking Differential

 

Concept

 There are many types of lockers available for landrover vehicles but all seem to have a weak point or Achilles heel, so we set out to design a locker from a blank canvas to be cost effective, strong and without the weaknesses of other designs.

  Design 

We wanted a selectable locker which would fit into the rover diff casing with minimal installation work, i.e, without mods to the axle housing. We looked at electric and pneumatic activation and decided pneumatic was preferable for reliability and simplicity. One of the key requirements for our design was zero air leaks as this is one of the main issues with other air operated lockers. To achieve this we opted for a static piston rather than the rotating seal used on other air lockers, which was prone to leaks. We also wanted to ensure we had a full 6mm of engagement when locked as other air lockers have just 2.5mm and are prone to slipping out of lock when loaded.

 The final design settled on a 4 pin diff centre; one of the diff side gears has an outer ring of dog teeth around it which a locking collar engage with when locked, thus locking collar is on the crown wheel side, the locking collar is held with 4 pins which pass through the diff centre on the non crown wheel side . Outside the diff centre the 4 pins are peened to a steel thrust ring, this assembly forms a robust all steel ‘cage’, not plastic like other designs which were prone to failing.

 On the non crown wheel side inside the journal end cap is a cylinder in which sits a bronze piston, the piston has a groove around the inside and outside diameters which O rings sit in. Once the piston is fitted a retaining ring is bolted around the end of the cylinder to stop the piston coming out, air is fed to the cylinder through a copper pipe to a 90 deg compression fitting which fits to the outside diameter of the cylinder. When air is applied the piston moves along the cylinder until it reaches the retaining ring, it does not spin in the cylinder, the cylinder is static and sits on a bronze bush at the side of the diff centre. Inside the journal bearing on the cylinder side is a bronze thrust ring to hold the cylinder. A locking tab is fitted to one of the journal end cap bolts and locates in a cut out in the cylinder to stop it rotating.

 

 

The copper air feed pipe goes from the side of the cylinder through the diff housing, it doesn’t have to pass over the crown wheel.

 The lockers are assembled on our premises and supplied with switch (front and rear fascias), solenoid, air line and all required fittings, excludes air compressor.

 We have used 6mm pneumatic push fit fitting to the locker as we have found that other air lockers that have threaded fitting and hoses will either tear a pipe or break a fitting if the air line gets snagged on a branch etc, our locker uses these pneumatic 'push fit' fittings which will still get snagged but just pull out so you just need to push back in to fix rather than having to replace hoses etc.

 Operation

 When air is applied, the static annular piston moves across and pushes the spinning thrust collar which slide the 4 pins and move the locking collar over the side gear teeth to lock the diff, the diff is unlocked when the air pressure is released and 4 return springs push the locking collar out of engagement with the side gear.

 

 Materials

 Whilst we want the unit to be cost effective, the strength is paramount, all steel parts are turned from billet UK spec EN24T, Gears are 8620, locking collar is 300M.

 Key features :

 Full 6mm of locking collar engagement

Static annular piston, minimises risk of air leaks

Large diameter piston gives a 55% increase of hold in force over competitors

High spec materials throughout

Competitively priced

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