Poka-yoke Gone Wrong

Polka yoke?

Poke a yolk?

Pokey yoke?

Po ka yo ke?

"Poka-yoke" is a way to prevent mistakes. One way to do this is to have engineers design parts that can't be assembled incorrectly. Or, in the case of aftermarket auto parts, they make parts that can't even be assembled at all.

Here's an example from a 2007 Ford Freestyle. I needed to replace the passenger side lower control arm because the ball joint was bad. [I'm replacing it with Mevotech part number CMS40148, made in China]. The boot was torn and that caused the ball joint to lose the grease and allow dirt in. Then wear happened and it got enough play that it needed to be replaced. 

The Ford engineers realized that there was a possibility that someone would try to swap sides and use one of the control arms upside down on the other side of the car. I'm not sure why someone would do that, but often times these types of "fixes" are because somebody on the assembly line actually did that. You know that one guy. 

So to prevent this, they added a little metal tab which can be seen behind the right side bolt hole in this picture.

This makes it so the bolt holes won't align if someone attempts to install the wrong side control arm. I can just imagine all these cars stopped on the assembly line with the lower ball joints upside down and they can't mount the wheel knuckles. You can't be machining parts on the assembly line, so this will prevent workers from making that mistake. 

So when I went to install the new control arm, it wouldn't go in. I tried to force it in, but it wouldn't align. I got out the rubber hammer and started whacking it. Still no luck. So I took it out to compare it to the old one. For some reason, Mevotech put the keyway in the wrong place.

You can see the mark from where the tab contacted it. That's nothing a little grinding can't fix. Now it fits!

Too bad Mevotech didn't error-proof their assembly process. But if they had, you wouldn't be reading this.