Continuously moving assembly lines can move as slow as they need to, to fit the work on the length of the line. However, they cannot be as fast as you like. There are some limitations on the speed of the line due to the limitations of the walking speed of a human worker, and even more so due to the ability of the worker to do work while the line moves. Let’s have a look!
A Quick Recap
A continuously moving assembly line moves the parts continuously. The workers move with the part (either by themselves or on a moving platform). Once they are done, they walk back in the opposite direction to their next location. The animation below illustrates this principle.
On the Walking Speed of Humans
Obviously, one key limitation in the speed of a moving assembly line is the human worker. So let’s first look at the speed of humans. The fastest human ever (so far) was Usain Bolt, who set the world record in the 100-meter sprint at 9.58 seconds. Obviously, this is not doable for your workers. And even if Usain Bolt works for you, it would not be sustainable. Besides, I strongly advise against running in a factory unless someone is in danger. Walking is much better!
The walking speeds of humans vary considerably. People with longer legs and a higher center of gravity tend to walk faster. Younger people walk faster. Rough terrains slow people down. Mashing everything together comes to an average walking speed on flat ground of 1.42 meters per second. In industry, however, the pace may be a bit faster. The Methods-time measurement system of predetermined motion assumes a walking speed of 1 foot every 5.3 TMU, which translates into 1.60 meters per second, not including turning (0.7 to 1.3 seconds depending on the footing) and assuming that the worker does not carry any load. Some companies in Japan even standardized this and have a timed walking distance to train their people on the proper walking speed. I also did this walk, and it is not a leisurely stroll but is quite doable. Obviously, the assembly line cannot move faster than the worker.
Hence, if the worker can move 1.6 meters per second, the line cannot be faster than that. But don’t you dare to set your assembly line to 1.6 meters per second, or you will end up with a system like Charlie Chaplin’s “Modern Times,” and your workers will go crazy. By the way, the assembly line speed in the Modern Times video clip below is just around 1.5 meters per second, or a brisk walking speed. You don’t need to watch the video to see that this is insane…but still watch the clip because it is a very good movie.
Type of Line
Here we have to distinguish between different types of moving assembly lines.
- For small parts the worker may be stationary and complete the task while the part passes through in front of him.
- For large- and medium-sized parts the worker may be moving along the line on his own two feet.
- For larger parts like cars there is often a moving platform that carries the worker (and his tools) along.
A moving assembly line where the worker remains stationary is possible but uncommon. A pulsed line is often better suited for that, since it allows the worker to work on the part without the part moving. If it is a moving line, the speed has to be slow enough to give the worker enough time and a part that moves only very slowly. Don’t do Charlie Chaplin “Modern Times,” please!
A moving assembly line where the worker walks along with the line (and later back again) is quite common, from automotive to forklifts and others. Usually they are for medium-sized or larger products like cars. Here, too, the line should be slow enough for the worker to comfortably do their work with little walking in between. Tool carts may be designed to move along with the line, and at the end of the station roll back to the beginning of the station.
With moving platforms you could have, in theory, quite a quick speed, but in reality there are problems with that. First, the worker has to get off the moving platform and walk back to the beginning of his station. At the same time the line zips past him. Since the worker is on the way back, the space cannot have any parts or work and must be empty. Hence, a high speed merely makes the line longer by having large gaps between the parts. Secondly, the worker has to get on and off the assembly line to get materials and tools. If the assembly line moves slowly at around 0.1 meter per second, this is easy to do. However, the faster the speed difference is between the moving platform and the (stationary) surroundings, the riskier it is to get on and off.
You surely remember stepping on a moving walkway or an escalator, having to pay attention so as not to stumble or fall. Now imagine doing this multiple times per minute eight hours a day, and it is almost guaranteed that sooner or later you will fall. Moving walkways usually move much slower than an average walking person and often have a speed of 0.5 meter per second or less. Moving platforms in assembly lines are even slower and often have speeds at 0.1 meter per second or less. This makes stepping on and off much safer. In theory the worker could also stay on the platform and walk back, but then he would have to walk against the moving platform too. Such continuously moving platforms are common in the automotive industry.
Speeds of Continuously Moving Assembly Lines in Automotive
The average length of a car is around 5 meters, although this varies considerably. At the time of writing, the Mitsubishi Mirage is the shortest available normal car with a total length of only 3.845 meters. On the other end we have the Rolls-Royce Phantom VIII EWB with 5.99 meters. Have fun parking that in an European city… but then, you don’t park your own Rolls Royce, I guess. If we look at pickup trucks, we also have monsters like the Ford Super Duty at 6.76 meters. Overall, most car assembly lines allow 6 or 7 meters per car on an assembly line.
The fastest automotive assembly line in the world is the Yichang Factory of GAC Motor, where a car is assembled every 46 seconds. However, fast is not necessarily good. The average value for automotive assembly lines is more around 60 to 90 seconds per car. The Audi A8 assembly line in Neckarsulm takes around 9 minutes per vehicle… but this is due to the limited demand of these high-end and ultra-luxury vehicles.
The Yichang Factory, assuming 6 meters between cars with a car every 46 seconds has a speed of 0.13 meter per second. More commonly, however, you have a car every 7 meters with a 60-second takt, giving a speed of 0.11 meter per second. For the Audi A8 line in Neckarsulm, I assume a distance of 8 meters (since they are long luxury cars and the distance may be even more) every 9 minutes, which gives a speed of 0.015 meter per second. For the casual observer that looks like the line is standing still and the motion is not really visible… except a minute later it has moved a bit. Please note that all of these speeds are a mere fraction of the human walking speed of 1.4 or 1.6 meters per second!
What Are Good Speeds?
Overall, most continuously moving assembly lines are moving quite slow, and a grandma with a wheeled walker would easily outpace the line. Speeds of 0.1 meter per second or even less are common. The line can be as slow as it wants to be to fit the work on the line, but it should not be much faster than 0.1 meter per second; otherwise the workers will find it increasingly difficult and stressful to work. In that aspect, a “fast” line is not necessarily a good line. I would rather have a slower but shorter line than a faster and longer line.
After five posts I think I have written everything I wanted to about continuously moving assembly lines (for now). A lot of this is also my original research, and I would be surprised if you find this anywhere else (if so, let me know in the comments). In any case, I hope this post on assembly line speeds was of interest to you. Now go out, set up your assembly lines, and organize your industry!
PS: This series of blog post was inspired by Sobha Modular.