Herman Miller Aeron: Replacing the Gas Lift

Having documented other people's difficulties with replacing their Herman Miller Aeron gas lifts, it was finally time to do the same for my own 1999 chair. What are the current challenges and tools available for this activity in 2022?

Since acquring my late 90s Aeron, most of the common parts have been replaced (sunken seat pan, plastic clamshell hip bolts, torn seat back, fixed arm rests, wobbly castors, torn lumbar support, seat pan edge foam insert) but whilst the gas lift had always been sticky and the chair wobbled, this was put off due to the reported difficulties. However more recently this was no longer an option to defer replacement since the gas lift would no longer operate, being stuck after lowering. But how easy and what options for parts are available.

Gas Lift Basics

Gas lifts in most chairs are a universal size with the following industry measurements/characteristics:

• outer (50mm diameter) tube length, V
• min and max length of total gas cylinder, L1 and L2
• extenion length of the telescopic piston (28mm diameter) including tapper to tilt mechanism, (L2-L1) Stroke
• outter tube tappered section into base, typically 60mm length, X
• projection from under base (below X), Y
• top button or side cable activation

The only signficant gas lift specification is the Class of the cylinder which dictates the weight it can support and the metal wall thickness of the tube and telescopic piston: Class 4 supports heavier loads (commonly quoted from 150kg-250kg) compared to Class 3 (up to 150kg). The key point being for the gas lift: given the weight of the Aeron chair itself and the support it needs to provide the user, you should be looking for a Class 4 gas lift - don't repurpose from an Ikea chair even if it does physically fit.

The OEM gas lift spec

The factory fitted original 2 stage telescopic gas lift I removed from a 1999 mark I Aeron had the following observations:

• outter tube tappers from 50mm to 47mm for section X, 210mm length V
• telescopic piston tappers from 28mm to 26mm, with 28.5mm inserts into tilt mechanism
• 250mm (10") stroke
• 40mm length projection, Y
• top button activation

It appears that Herman Miller have used a gas lift with industry standard specifications which makes sourcing a replacement much easier.

The type of gas lift activation is imporant as the tilt mechanism uses different hardware depending on type and the two types are not compatible: pre 2012 top button activated, post 2012 side cable activated. Identification is relatively easy after you pop the top cover of the tilt mechanism and visually inspect. Button activated chairs/cylinders have a butterly-like activation mechanism wth a lever fitted over the activation button that plugges the lever when the chair's right side actuator tab is pulled.

Replacement Process

Unless you are absolutely sure that the gas lift is defective, I would recommend that you first verify that you do indeed require a gas lift replacement before attempting given the potential cost, trouble and damage you can cause. One common problem with gas lifts is that users report their chair will sink slowly as they sit. This can be definitely a defective gas lift but it can also be an inadvertant gas lift activation for button activated chairs: as you sit, the extra weight subtly presses down on the actuator button and thus the chair sinks.

Alternatively, a similar related problem may be if chair does not raise nor fall when the gas lift actuator is pulled


In both cases you should check the following: the pre-2012 Aerons actuator button are under a butterfly mechanism where the base of the butterfly mechansim's lever is a 4mm hex set screw adjustment ; adjustments to this raises or lowers the lever when it is in its reseting positon - this should be adjusted so that the lever has enough tension/no bounce to it when pressed with your finger and this will also result in the actuator pull tab having no slack.


^{(c) Herman Miller: green - lever, blue - 4mm set screw adjustment}

If after this set-screw adjustment has been completed but the gas lift is still not operating correctly then you know its the gas lift.

Replacement Process and Tools

The internet's recommendations for removal would involve either:

• a specialised lift off tool that is attached to the telescopic tube butted up against the tilt mechanism and hammered until seperation
• exposing the top of the gas lift in tilt mechansim, with removal of seat pan and any activation mechanisms and to hammer out the gis lift via a special diameter pipe
• pipe wrench to twist off the cyclinder by gripping the telescopic tube

whereby the top 2 have been mentioned in Herman Miller's service manuals.

However in the last 3years or so there is a further option which involves a simple two metal ring removal tool: one ring is tighted and clamped to the 28mm telescopic tube and screws are turned to push against the second metal ring which is loosely attached around the same telescopic tube whilst pressed against the tilt mechanism - the extension of the screws will gently and slowly push the tilt mechanism away from the telescopic tube and finally separating the two.

^{(c) Office Oasis}

Using the 2 metal rings is best done with your chair toppled over, with the back facing upwards as the same rests on the ground - this leaves the base up in the air.

In the UK a kit consisting on the rings and a Class 4 single stage gas lift was available from an American firm as Office Owl universal gas lift with removal tool for about 30 GBP: the (reusable) metal ring removal tool was being sold elsewhere for 20 GBP. In the US market, there appears to be an identical set from Office Oasis

This removal method also requires that the 28mm telescopic tube is accessible. For me, this was not the case - having previously tried and failed to using a tongue and groove pliers (aka water pump pliers or grips) (rather than the min 24" recommended pipe wrench that I don't own) to twist off the gas cylinder, the chair had sunk further. With the chair positioned with the base in the air, I had to use the same tongue and groove pliers to grab hold of the wider/2nd stage telescopic tube to twist and pull to extend the 28mm telescoing tube which appeared to have been gummed up with 20+ years of dried up lubricant.

I would definitely recommend separating the tilt mechanism first since separating the 50mm tube from the base requires brute force hammering the tube from under the base. When I hammered the tube out from the base there was quite a lot of damage to the gas cylinder - if you opt to separate the base first and then, for whatever reason, are unsuccessful with the tilt separation, then you have no working chair: having a failed tilt separation first, you can still use your chair with the same issues you have prior to your removal attempt.

A great video showing the removal process using a pipe wrench for top and side activated gas lifts is available from this US-based office refurb seller

Installation is simple: insert the new gas lift into the base and then the tilt mechanism on top - gentle sit in the chair to seat everything; one nice upshot aside from the working gas lift was that the new cyclinder removed the slight wobble of the chair

Conclusions

Whilst the gas lift removal is still not the easiest Aeron maintainence task, it is now significantly improved with the options and breathes new air into your aging Aeron.