Please clarify this gear problem

[mechgirl]

#538 in Mech Systems depth of the 2008 NCEES sample exam:

A 42-tooth pinion running at 1800 rpm drives a gear at 1080 rpm. The spur gear pair is to be enclosed in a cylindrical casing with a total diametral clearance of 1/2". The diametral pitch is 16 teeth/in. The minimum required casing diameter (in.) is most nearly:

The solution starts with "Let P=6" Then goes on to solve using P of 6, while I used a P of 16.

Can someone please tell me why the given diametral pitch of 16 was not used in the solution? Where did the diametral pitch of 6 come from?

 

[BrianC]

It doesn't make sense to me. It seems like the "16" is a typo based on the solution and answers given, but sometimes assuming a typo is the easy way out. I don't see any reason for what they did.

 

[mechgirl]

It doesn't make sense to me. It seems like the "16" is a typo based on the solution and answers given, but sometimes assuming a typo is the easy way out. I don't see any reason for what they did.

I have such a difficult time with machine design... but I'm glad someone else thinks it's a typo. I did find another typo in the thermal and fluids depth section. Enthalpy was given as h= lbm/hr . Hope there aren't typos on the test.

I will say I'm much more confident after working through the NCEES mech materials and thermo/fluids depth sections. I did nearly all the fluids problems right, and a good portion of the machine design were correct, but did take much longer than 6 minutes This week I'll work through the HVAC 6 min solutions, and when finished, I'll take the NCEES practice exam morning and HVAC sections timed.

 

[Solar_Chris]

Problem #338 is a typo. The errata for the 2008 Mechanical practice exam can be found at this link.

2008 NCEES Mechanical Errata

The Errata for the 2001 Practice exam can be found at this Link