hard drives are most common at 7200rpm
so are many engines that are limited to around 7000rpm?
is that on purpose because of some physical limitations or just "because they other guys did it" seems like a good idea

 

The comment on hard drives is inaccurate. There's plenty of 5k, 9k, 10k, and 15k hard drives out there. Then, there's the fact that most diesel engines operate under 3,500 RPM's, and older engines couldn't push much past lower RPM's as well. Harmonics are an issue and so is bearing load, but the comparison you're making isn't comparing apples to apples. If that were the case, then all electric motors and combustion engines would hover around the 7k mark redline. You're making a rather large leap, that's really not there.

 

not so much a matter of the limit of physics as it is a byproduct of similar design constraints / intended uses.

sport bikes tend to be higher 10-14k rpm. they have different constraints and intended uses.

lawnmower/small engines (4 stroke) tend to be lower 4-5k rpm. again, different constraints and uses.

some input parameters that dictate the theoretical limit are mean piston speed (factors: durability requirements, stroke/torque curve, cost to build and operate), valve spring stiffness (factors: cost, desired torque peak, fuel economy), total cross-sectional area of intake and/or exhaust path.

 

was just curious if there was any connection between two .

 

It's up to the engineering team; every mechanical device is a series of compromises.

 

between a hard drive and an engine the only similarity is that they both spin.

hard drives are BLDC motors I believe. like automotive gasoline engines they are also subject to design constraints such as cost, performance, size, ect. but the similarities end there.