If a chair fails in the field, the gas lift is near the top of the suspect list — and it is the single part a price-driven supplier will quietly swap to save a few cents, because the buyer almost never inspects it. So when we quote a gas-lift cylinder, the class and the stamp come first, not the unit price.
What "class" actually refers to
Chair gas springs are graded under DIN 4550, the German standard most cylinder stamps reference. In practical duty terms, a Class 3 cylinder suits standard task and study chairs used a few hours a day; a Class 4 is built for harder service — call centres, hot-desking, 24/7 control rooms and heavier users. The class is not only about a maximum weight. The higher class also tolerates far more up-and-down strokes before the cylinder starts to sink slowly on its own, which is the failure mode users actually complain about.
The geometry is shared across classes and across makers: outer tube Ø50, piston rod Ø28, mounted on the universal column taper. That is deliberate — it is what lets a chair factory keep one base and one mechanism and just change the cylinder class by line.
Read the stamp
A genuine cylinder carries a mark rolled into the steel. A good one reads something like "SGS · BIFMA X5.1 · EN 1335 · LGA TÜV · DIN 4550 · Class 4." That string tells you which test houses and which standards the cylinder was approved against. A bare cylinder with a class printed on a paper sleeve and nothing on the metal is a class claim, not a class. Ask any supplier — us included — for a photo of the stamp before production. We would rather you check than take our word for it.
The trade-off, stated plainly
Class 3 is fine, and we ship a lot of it. For a home-office chair that gets four hours a day, paying for Class 4 buys a margin the user never reaches. But for a contract order going into an open-plan office where every chair gets sat in by different people all day, we push Class 4 — and yes, it costs more per unit. The reason is warranty math, not comfort. The few cents saved on Class 3 across a 40-foot container come straight back as replacement cylinders, spare-part freight and an annoyed reseller. We have watched buyers learn that the expensive way.
You do not have to upgrade the whole catalogue. Default to Class 4 on the chairs that carry your brand and keep Class 3 on the budget line. Where a fatigue or temperature report is needed, the cylinder class is part of what we book into testing — we build to DIN 4550 / BIFMA / EN methods and testing can be arranged. The how is on our quality and testing page.
Tell us where the chairs go and how hard they will be used, and we will map class to duty and send the stamp scope up front. Reach us via the enquiry form or ztjdxs@hz-zhongtai.com. The full parts list is on the catalogue.