16 Mechanical Design Failures Told by a Former Designer: Lessons Learned from Drawing and Selection Errors

16 Mechanical Design Failures Told by a Former Designer

Lessons Learned from Drawing and Selection Errors

In the work of design, even if something is perfect on paper, unexpected problems can arise in the field.

I myself have experienced many failures in the field of mechanical design.
Drawing errors, calculation oversights, equipment selection mistakes, and on-site installation troubles.

At the time, these were all events that made me break out in a cold sweat, but looking back now, they were alllessons learned as a designer.

This time, I will look back on my own bitter experiences and organize and introduce design mistakes.

I am embarrassed to say they are all elementary, but I would be happy if this serves as a reference so that those of you who are just starting out in design do not make the same mistakes.

1 Drawing Errors

First, there were many mistakes related to drawings.

・Inconsistent reference dimensions

The way reference dimensions were taken was not unified across different sections of the drawing, leading to confusion regarding the dimension references.

A drawing is the embodiment of the designer's thoughts.
If the references are ambiguous, the production side will also be confused.

・CAD dimension discrepancies

There was a time when I issued a drawing without noticing a dimension discrepancy in the CAD drawing, resulting in parts being manufactured with incorrect dimensions.

The size of the part drawing itself was correct, but the dimension display was wrong.

・Scale errors in old drawings

When I reused an old drawing and pasted it into a part drawing, the original drawing was drawnnot to scale.

I once issued a drawing without noticing that.

・Assumptions about opposite-hand drawings

When I created a drawing for the opposite hand, I assumed everything was just mirrored and neglected to check the details.

In reality, the positions of some parts were different.

・Overlapping dimension lines

I once made a mistake in the placement of duplicate dimension instructions, causing them to overlap with the extension lines, which made the drawing extremely difficult to read.

・Forgetting to design tool clearance

Although I specified male thread machining, I failed to provide a tool clearance in the corner.

As a result, it was discovered during assembly that the length was insufficient. Furthermore, the thickness of the bearing nut also ended up being a special size.

2 Failures in Calculation and Equipment Selection

Even scarier than drawing errors are mistakes in calculations and equipment selection.

・Lack of experience with safety factors

There were no problems in the calculations, but an unexpected radial load occurred, causing the bearing housing to break.

The cause was that the safety factor setting was too lenient.

・Selection error for guide rollers

When I used commercially available casters for the guide rollers,

・Thrust load
・Allowable peripheral speed

they did not meet the conditions and failed prematurely.

・Bearing tolerance error

The bearing tolerance was not appropriate, leading to a situation where the shaft had to be chrome-plated and re-machined.

・Insufficient number of V-belts

I used V-belts for power transmission in a crusher, but because the number of belts was insufficient, premature wear occurred.

・Motor capacity selection error

As a result of selecting a motor without considering the moment of inertia, the operating current exceeded the allowable value.

3 Lack of design consideration

Even if drawings and calculations are correct, a lack of design consideration will cause problems in the field.

・Insufficient maintenance space

Space for bearing replacement was not secured, resulting in a structure that required the entire device to be disassembled for replacement.

・Forgotten design of adjustment bolts

Due to the omission of adjustment push bolts, centering and assembly adjustment of the rotating body were difficult. To prevent recurrence and maintain accuracy, we decided to additionally install knock pins for fixing the bearings.

4 Lack of understanding of the operating environment

The performance and durability of a machine are greatly influenced by the environment in which it is installed. Previously, when we used a standard commercial electric motor for a ship, the motor suffered early failure due to unexpected acceleration (G) unique to being on board. Consequently, it was found that the design margins for the bearings and reduction gears were also insufficient, and we ultimately changed them to specifications two ranks higher.

5 Installation and construction troubles

When we used a box-out foundation for the installation of a screw conveyor, a larger-than-expected deviation occurred. This was because the anchor bolts tilted, preventing the hex nuts from being tightened all the way. I learned the importance of not only designing according to the drawings but also considering the accuracy of the construction site.

6 Insufficient environmental measures

In the past, there was a trouble where the inside of a heat exchanger casing became flooded because condensation measures were not considered.

Furthermore, in winter, when we adopted a high-efficiency heat exchanger, condensation and freezing occurred on the outlet side, blocking the air path and leading to insufficient air volume. Currently, to prevent freezing, we are forced to operate by intentionally lowering efficiency to raise the outlet temperature.

Conclusion

Designers grow through 'failure'

Looking back, there are countless design failures.

However, as those experiences accumulated, I developed an intuition for
“this is risky”
and
“this needs to be verified.”

In the world of design, it is often said:

“Drawings do not lie. However, designers make mistakes.”

That is precisely why

・Verify
・Record experiences
・Share failures

I believe these three things are essential.

I would be happy if this article serves as a small hint for those who will be involved in design in the future.

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