Multi-Station Circumferential Seam Welding Machine for Shock Absorber Tubes

A Shock Absorber Tube Lives Or Dies On One Seam: Two Copper Discs Roll It Shut While The Table Indexes

Application story  ·  Multi-station circumferential seam welding of shock absorber tubes - rotary table, servo clamping, servo head height, medium-frequency DC power

A shock absorber body is a steel tube full of hydraulic oil, and the joint that closes it is a circle. That circle has to be welded closed all the way round - no pinhole, no cold lap, no place where the discs lifted - because one leak is not a rework. By the time the seam is welded the tube is already filled and closed, and a tube that weeps is scrap. Working that circle one tube at a time on one station is exactly where the cycle time goes: the weld itself is a few seconds, and everything around it is waiting.

Multi-station seam welding machine: four-column frame, three welding heads over a rotary table, guarded work zone, control cabinet at the side

The Seam Is A Circle, And It Has To Close

The part is a drawn steel tube with an end cap and a mount ring at the top, and the seam sits between the ring and the tube wall. Its length is the circumference - which means the bigger the tube, the longer the seam, and the longer the weld has to stay stable. On a tube of this size that is a few hundred millimetres of continuous weld that has to be made without a stop.

Stopping matters more than it sounds. A seam weld that starts and stops leaves a lap at the restart, and a lap on a pressurised tube is the first place a leak appears. That is why the process here is a seam and not a row of spots: the discs come down once, the part turns once, and the weld is one closed ring.

Welded tubes: the ring seam under the mount ring has to close completely - the tube is oil-filled, so the seam is a seal as much as a joint

Three Stations Turn While One Weld Is Made

The tubes stand on a rotary table with three fixture positions. While one position is inside the guard being welded, the operator is loading the next tube and unloading the finished one at the other two - not waiting for the weld to finish, and not standing in front of an electrode. The table indexes, the heads come down, the seam is welded, the heads lift, and the table indexes again.

That overlap is where the time is recovered. Compared with the same welding head on a single station, the multi-station table is quoted at roughly twenty percent shorter cycle, and the saving comes from the loading time being hidden inside the welding time rather than added to it. Loading still takes as long as it takes - it just stops being on the critical path.

Three tubes standing on the rotary table at the three stations: the centre one is under the heads, the outer two are at load and unload

Two Copper Discs Pressing On The Same Seam

The welding head is not a single electrode. Two copper discs, roughly two hundred millimetres across, come down from opposite sides onto the seam, and the current goes in through one disc, across the ring joint and out through the other. Between them the part turns, so the discs roll along the circle instead of sitting on one spot - the contact face keeps changing, the disc wears evenly, and there is no crater to dress out after every few hundred tubes.

Each disc runs in its own pneumatic slide with a pressure regulator, so the force on the two sides can be matched to the seam rather than assumed equal. A tapered centre spindle comes down inside the tube mouth to hold the part concentric while it turns; without it a tube that is a millimetre off centre puts the seam a millimetre off under the disc, and the weld width varies around the circle. Blue articulated coolant hoses are aimed at the disc rims, which is where the heat actually is.

The head up close: two large copper discs on their pneumatic slides, coolant hoses at the rims, and the tapered centre spindle between them

In the weld position: discs down on the seam, centre spindle seated in the tube mouth, secondary copper behind the discs

Servo Height, Servo Clamp - One Program Per Tube

A shock absorber family is not one part. The same platform comes in several lengths and several tube diameters, and on a fixed machine every one of those is a shim, a spacer or a different set of jaws. Here the head height is servo-driven, so when the tube gets longer the heads move up by the difference; the clamping is servo-driven too, so when the diameter changes the jaws close to the new diameter instead of being swapped.

Both are positions in the program. Changing from one tube to another is a program change - height, clamp stroke, weld schedule and rotation speed - which is the difference between a machine that runs a family and a machine that runs one part number.

Two tubes from the same family, different length and different diameter - the change between them is a program, not a fixture swap

Machine front: the two servo drives for head height sit on the top beam, guarded work zone below

Medium-Frequency DC, Not AC

The power source is a medium-frequency inverter DC unit rather than a conventional AC transformer. It draws a balanced three-phase load with a high power factor, which is cheaper to run and does not unbalance the shop supply; and because the output is DC there is no current zero crossing in the middle of the weld, so the heat into the seam stays even instead of pulsing twice a cycle.

On a thin tube wall that is the difference between a seam and a burn-through. It also makes the welding transformer smaller and the secondary loop shorter, which on a multi-head machine matters - three heads inside one frame do not leave much room for heavy copper.

The machine in the customer's plant: three heads on the top beam, stainless guard over the work zone, control cabinet and power unit at the side

A Cell, Not A Standalone Machine

In production this machine does not stand alone. It sits in a line with the stations before and after it - the assembly that puts the cap and ring on the tube, and the stations that follow - and the guards, the control panels and the service side are laid out so that one operator can work the cell rather than one machine.

That is also why the control panel is on a swinging arm rather than bolted to the frame: from the loading position the operator can see the screen, the fixture and the weld zone at the same time, instead of turning his back on the parts to read a cabinet.

The line: this machine with the assembly station ahead of it, each with its own panel on a swing arm

Seam welder and the next station side by side - guards and service access arranged for one operator across the cell

The Same Seam On A More Complicated Body

The parts are not always a plain tube. The same machine welds bodies with a side spigot, a bracket lug and a flange ring on the wall - each of those is another circular joint, taken in the same way: part clamped, discs down, one turn, closed. The difference is the program and how many rings it has to make, not the process.

That is the practical test of a dedicated machine: when the next variant arrives with one more boss on the side, the answer should be a new program and a modified fixture insert, not a new machine.

A more complex body from the same family: side spigot, bracket lug and flange ring - all closed with the same circumferential seam process

Key Specifications

Where It Welds

•   Automotive and motorcycle shock absorber bodies - the ring seam under the mount ring or end cap

•   Damper tubes, gas springs and suspension reservoirs where the closure has to hold oil or gas

•   Hydraulic cylinder barrels with a welded end cap or flange ring

•   Bodies with a side spigot, bracket lug or flange ring - one closed ring per joint

•   Any family of thin-wall tubes where length and diameter change but the seam process does not

Heads, fixture, clamp jaws and programs are built from the customer's tube drawing and a sample. We weld the first parts on the real tube and set pressure, rotation speed, current and schedule before the machine ships.

How A Custom Machine Is Built

A dedicated seam welder is mostly a fixture problem with a welding head attached, so the sequence starts with the part and works outwards:

01   Tube drawing, cap or ring drawing, and seam position review

02   Seam trial on sample tubes - closed ring, weld width, no leak, no burn-through on the wall

03   Process proposal - number of stations, head arrangement, power source, cycle estimate

04   Fixture, clamp jaws, centre spindle and disc design in 3D

05   Frame, rotary table, servo axes and pneumatic/water circuit build

06   Medium-frequency power source and control integration, program per tube variant

07   Assembly and run-off on customer tubes before shipment

08   Delivery, installation and commissioning on site

09   Operator training in program changeover, then after-sales service and spare discs

Talk to Pudian Technology

Guangdong Pudian Automation Technology Co., Ltd builds resistance and laser welding equipment in-house - transformers, controllers, discs, fixtures and complete machines - one team, one solution. Send us the tube, the cap or ring, and a leak-test requirement, and we will weld sample parts and send them back with a parameter sheet.