Build Ergonomics

Intro

Build ergonomics - the level of effort for a build, can be evaluated meticulously. We need to define:

1. Number of steps
2. Time involved
3. Difficulty

Build Ergonomics For Dummies

To do a baseline quantification of build ergonomics:

1. Record the number of steps - defined as a single operation with 2 hands. Include every single step. The assumption is that every step is about a minute.
2. Record the weight handled in each steps, in multiples of pounds or kilograms. Assume 1 lb or kg for any light object.

The baseline quantification is step-pounds or step-kg. For example, 250 steps, where no heavy object were lifted - would be 250 step-pounds as the ultimate quantification of ergonomics.

This is a first-order approximation of build ergnomics - which can be compared to other cases. For example, in a case where we build a 5000 lb tractor in 2500 steps - and a lot of the parts are 50 lb - then we can make certain determinations:

The number of steps defines how much can be done. The weight factor determines the level of fatigue.

The human fatigue limit may be about 20,000 step-lb.

Number of Steps

Any single step can be defined as one operation with 2 hands. For example, take a nut and insert it into another 3D printed piece. That is One Step. Or, take a nut and put it on a bolt. That is one step. Done manually - that is a minute - done with an impact wrench - it's seconds.

But to get the actual effort - we must consider how long one action takes. Thus, it's useful to quantify in terms of step-minutes. A step that takes one minute takes one step-minute. A step that takes 15 seconds is a 1/4 step minute.

So with every step, we should identify cases where a step takes significantly different time than one minute. Assume that one minute is the average time. But some steps may require wait times, or just require a lot of a specific operation - such as threading a nut manually down a long threaded rod - which can take minutes. So measuring time for long steps is useful - on top of counting the steps.

Time of Build

Counting the number of steps as such gives an idea of build complexity. A build with 3525 steps - with a minute per step, would take 60 hours to complete. That is a realistic estimate of, say, a tractor build.

About one minute per step is a reasonable, rough estimate for many actions. This principle is an effective way to assess overall build time: simply take the number of steps: say 254 - and guess that it it took ~4 hours to do a build.

To reduce build time - we can assess each step - and see what optimization can be made: such as using power tools, CNC tools, or redesigning for simplicity. The last point has the most impact on build time: it separates good design from bad design.

Weight: Difficulty of Build

In terms of human stamina - the most important determinant is weight of the build. Lifting 1 kg 1000 times is much easier than lifting a ton one time. Heavy objects fatigue a builder - such as when building a house or a tractor.

Jigs and hoists can provide mechanical advantage to make the work easy. Self-supporting design can make it easy: when you don't need to continue lifting/holding something to perform an operation, but instead do it on the floor, or on a support so that the only difficulty is moving the item into place.

Difficulty can be quantified, simply by totaling the pound weight of all the materials - every time they are handled or moved. For example, for a 3D printer, which ways 25 kg - the total weight lifted may be 250 kg - if you consider moving a certain part through multiple steps. For example, 1 kg of the frame may be moved 10 times, adding 10 kg to the weight. The ultimate limit of this example is 25 kg - where each part is moved only once.

The difficulty can be further clarified by multiplier for holding/moving things. For example - just lifting the object, which takes on the order of 1 second - is different than lifting and holding that object for 1 minute. Holding that object is then roughly 60x harder - as each second of holding is roughly comparable to one second of lifting.

Multiplying weight by time results in weight-seconds - a useful ergonomics measure. Thus, an useful measure would be weight-seconds. If a person is using a forklift to lift an object, the effort there is only the force equivalent of pushing a hydraulic control lever for the forklift, etc.

For example, lifting 1 kg has a kg-s effort, while holding it for 60 seconds has a 60 kg-s effort.

Quantifying Build Effort

We can now produce a decent, absolute and relative measure of build effort. This can help us determine - how much can be built in how long?

The absolute measure is Number of Steps, quantified as the simple count of Individual Steps.

Complexity

Design determines complexity of build. Complexity can refer to the number of build steps required to build in a certain function. If a designer pays extreme attention to build complexity, then they will design something for easy manual assembly. Besides design - complexity can refer to tooling. Are simple, accessible tools being used? And most importantly - is it a simple design? That means: the simplest way to accomplish a function - in terms of materials, geometry, or principle of operation.

Part of complexity - is access to documentation that reduces complexity. For example, having easy-to-access tools and instructions for deconstruction.

In today's world - sleek-looking black boxes dominate the product landscape. What about designs that are designed for easy repair? The sleek-looking black box is good design - if it can be repaired/reused. For example, it appears that Tesla cars are not good in this regard - see Tesla Repair.

Human Capacity

• A human capacity should be able to lift their own weight 100 times in a day, roughly.
• This means 5000 kg lifted per day for a 50 kg person - a long, hard day of slave-labor like proportion.
• If a person lifted 250 kg equivalent, that would be an easy day.
• Assuming a person can do one operation per minute - a person can do 500 operations per day. Thus, a 250 step procedure, with good workflow - should be done relatively easily in a single day.

For example, a 5000 lb tractor assembled in one day with one person - is about the limit of what a person can do assuming only one movement per part. Teams of 12 make this easy, making the lifting limit per person down to 1000 lb or so with couple movements per part.

For the house - 120 lb

Links

• OSE Instructional Guidelines