By Don Weisman
Table of Contents:
Chapter I. Manual height adjustment office swivel chair mechanisms.
Chapter II. Gas operated height adjustment office swivel chair.
Chapter III. Manual and Gas Hubs.
Chapter IV. Manual and Gas Bases, 4 and 5 legs. Removal/Install.
Chapter V. Castor types. Removal/Install replacements.
Chapter VI. Placement, adjusting and attaching replacement mechanisms.
~~~~~
There is no mystery in repairing Office Swivel Chairs that are intended to be
placed into service. Since most components are metal, worn and damaged parts
normally require replacement rather than repair. Few parts are actually
available, due to manufacturers wanting to protect themselves from product
liability lawsuits. I have personally been involved as an Expert Witness due to
an after market repair. Owners and professionals alike though well intentioned
can unwittingly cause errors, some causing bodily injury.
Print this exploded view drawing of chair parts for nomenclature and reference purposes.
Fig 1
General age can be identified by:
a. having 4 legs and a cast iron mechanism-Old
up to about 1940
b. having 4 legs and a steel mechanism-Usually from 1950 and later
c. having 5 legs and a steel mechanism-Usually from 1985 and later
A mechanism may also be identified by looking at the post bottom (nearest to the floor). It will often have the construction month and year stamped into the steel.
Fig 2
Fig 3
Fig 4
Fig 5
New cast iron replacement parts are not available anywhere! Some shops choose
to weld broken parts in order to put the chair back into service. This is not
always a service, but instead can be a disservice. Most often if you follow the
chair’s trail, you will probably find the softer metal (brass) used for the weld
(brazing) cannot take the stress or strain and has again broken in a relatively
short time. It is uncommon to find an old cast mechanism that is not loose and
wobbly. However, due to the very heavy (and beautiful) castings used, many are
still in good condition. Some cast iron mechanisms had oversized height
adjustment threaded Posts (1 1/8" to 1 1/4"dia.). Therefore, installing only a
new replacement steel mechanism may not be all the parts that are needed. The
industry standard since cast iron days are for a 1” post, and that may be too
small to fit the original base hub.
Hub replacement for cast iron mechanisms:
There are two types of hubs. Tube type and Dovetail.
Tube types do not employ a dovetail but depend upon sheet metal for strength. Tube types are also difficult to locate. The fact is; Tube types will not hold up well under a load. They become loose and wobbly and there is no repairing of the damage. Hub Liners for tube and dovetail types are available. It is rare to find a broken dovetail type hub, excluding the plastic hub liner. It is noteworthy here to mention, that Hub Liners do not contain threads for screwing in of a post. The threads seen and felt have been user worn pressed into the plastic by the steel post and may in fact be the only defect with the hub. The space created within the threads of the liner will permit a remarkable amount of looseness. It must be replaced. For an occasional removal, a professional hub removal tool is not necessary but often appreciated. A removal tool can remove and install a hub liner in about one minute.
I have found that about half the time, older wood leg dovetails fit new dovetail steel hubs with little if any fussing. Non-dovetail cast iron hub types will require dovetails to be created in the wood if changing hub types. With a new mechanism and hub, the old chair will now be as good as the day it was built and still retain it’s old world appearance and function.
The most common cause of a wobbly or rocky cast iron or steel office swivel chair is a loosened mechanism post. Welding is not recommended. The second most common rocking cause but seldom observed is that of the steel surrounding the pivot pin. The user has worn away steel creating an enlarged hole and metal filings on the floor. Looseness in the pivot pin area is difficult to observe, but stressing the base from side to side by using two base legs as a fulcrum or gripping area, tests the integrity of the pivot pin area. So when investigating a rocking complaint, it is important to determine whether the mechanism, or hub or both are at fault.
A Collier/Keyworth mechanism is the mechanism most often encountered when working with a 4-leg office swivel chair. Collier/Keyworth is a well-respected manufacturer of office chair mechanisms going back to 1906. Easily recognized by the dual spring configuration. Modifications were made over time, but the basic 2-coil spring design has been retained. In fact and up to present day, quality office chair manufacturers will incorporate C/K spring (and gas) mechanisms in their line, though 5-leg bases are now used instead.
Note that figure 6 utilizes a small 2 ¼” Bell. The smaller bell is intended to be used with a 5-leg base. The Bell serves as a height adjustment. Were it intended to be used with a 4-leg base, the bell instead would be about 4” in diameter. Aside from appearance, either bell may be used without affecting the chairs operation or ability to swivel and tilt. This is important to know, as replacement mechanisms (with little exception) are available in many sizes but only having small bells. Replacements with large bells are very limited in size requirements.
As in the cast iron mechanism (figures 3 through 5), steel mechanisms also suffer from the same loosening of the post and or enlargement of metal at the pivot pin area.
As mentioned earlier, welding of the post should only be considered as a temporary fix and may not entirely stop the rocking. The pivot pin area is usually also worn enlarged which contributes heavily to rocking and leaning problems. Keeping these two causes of a rocky wobbly chair in mind, it is always more cost effective to replace a mechanism when possible, solving all or both problems.
The only replacement mechanism parts available are the Bell, Threaded post and Thrust washer.
BELL:
A bell cannot be removed by unthreading off of the threaded post. The keyed washer located under the bell prevents the bell from being removed from the bottom of the post. As an alternative, it may be removed by removing the post from the mechanism. If the post is tightly seated, be careful not to permanently damage the mild steel striking area of the post while removing. Doing so may make it impossible to remove. If instead the post is loose and easily removed, the area in the mechanism that grips the post probably has worn away. Even if fill welded back, the missing metal can make it difficult to hold and be strong enough to be a lasting repair.
THRUST WASHER:
The thrust washer’s purpose is to prevent unwanted winding of a chair while it is in use. This too is another very common complaint. The thrust washer is either broken or missing and must be replaced. Three different types of thrust washers may have been used.
1. Steel, hardened: Only removable by post removal or grinding away the hardened steel washer.
2.
Two
part plastic thrust washer: (dark brown ring and white center).
This
type washer removes easily by unsnapping the two-part assembly. A white plastic
tip will be imbedded into the keyway and should be discarded. This type assembly
has sadly been made obsolete and the molds were destroyed in about 1995. I
suspect a liability lawsuit initiated that decision. The 2-part washer assembly
permitted amateur and pro alike to easily remove the bell or thrust washer.
Perhaps the cause of an after market injury and lawsuit. However, it did make a
very simple repair to winding problems when the thrust washer tip was broken
off. Thrust washer tips break when the user raises the chair beyond its stop.
3. Plastic thrust washer: (all white). Today, this is the only washer, which can be installed without removing the bell or post. The edge must be cut (sawn) before installation and then the tip can be twisted into the keyway, solving the winding problem.
The most difficult problem encountered when installing a replacement mechanism is, exactly where to screw it. Although covered in Chapter VI, the pivot pin location is the key to comfort and proper positioning. The location of the threaded post as commonly thought, is not the determining factor. Also, the user’s preference (position of recline) may demand a different pivot pin location other than the original (described in Chapter VI).
When a mechanism is properly installed (balanced), the user’s weight is not an issue (balance IS an issue). BIFMA/ANSI http://www.bifma.com/standards/ testing is for chairs supporting a 220 lb user minimum. Approved mechanisms will permit actual use of heavier loads, however longevity will then become compromised.
Next, Chapter II.
Gas operated height adjustment office swivel chair repairs