Algor Design Check

[Steve Cole]

This is a question regarding the Algor Design Check application included in Ironcad 8.

When you assign the material in the analysis mode you can view the material's properties.

Can anyone explain "mass density", it's units and how it is derived from the materials density?

MatWeb, where Algor gets it's material specifications, does not give this property on their spec sheets.

[Mike Twining]

Without knowing anything else I would assume it is the "density" of the part expressed in mass units as opposed to weight (so if you are using the US system you know if you are looking at pounds-force or pounds-mass). I love using the same title for 2 different measurement concepts . Its the kg/N difference in the U.S. system.

 

In this case, mass density would be "mass per volume" (as opposed to "weight per volume")... the difference being gravity.

 

MikeT

[Steve Cole]

 

Without knowing anything else I would assume it is the "density" of the part expressed in mass units as opposed to weight (so if you are using the US system you know if you are looking at pounds-force or pounds-mass).  I love using the same title for 2 different measurement concepts .  Its the kg/N difference in the U.S. system. 

 

In this case, mass density would be "mass per volume" (as opposed to "weight per volume")... the difference being gravity. 

 

MikeT 

11064[/snapback]

 

 

 

Thanks for the response Mike. Thats how I viewed it but I'm having trouble wrapping my mind around the units given in their table. I don't think that its all that important since the mass of the object shouldn't have any effect when doing a static analysis of the part. Static analysis is all that Degign Check does. the mass would come into play when doing a dynamic analysis, acceleration, deceleration, impact.

I'm just curious since I'm unfamiliar with the term "mass density" and it's units.

 

SteveC

[Mike Twining]

Ahh yes.... "Mass Density" is a term that gets shoved down your throat quite a bit these days when you go through Engineering school in the US (not to say that you didn't, but as I still live off pasta and top-romin I would consider myself somewhat fresh out of school). The units are basically "lbm (pounds-mass)/volume (in^3, ft.^3...whatever units you are in) OR "sluggs/volume". I gueess there has been so much confusion over "pounds" (i.e. "Is that force or mass?") that they like to make sure you know what you are doing. Anyway, the confusion should all be set to rest when the U.S. comes to their sences and goes metric.... until then, you get to keep your standard AND metric tool set.

 

To make thing weirder.... lbm=lbf (mass pounds=force pounds) on earth...so lbm is not a true synonym for kg.

 

kg = N/g <--"1 kg= metric mass which will move 1*m/s^2 when a 1N force is applied)

lbm = lbf/(normalized g) <--"1 lbm = standard mass which will move 32.2 in/s^2 when a 1 lbf is applied"

sluggs = lbf/g <--"1 slugg = standard mass which will move 1 in/s^2 when a 1 lbf is applied"

 

So... 1 slugg = 32.2 lmb....so you better know which units the program is expecting or you are going to have another mars polar lander on your hands .

 

2nd thing: I don't know what you are analyzing, but Mass Density can come into play on a static analysis in the form of weight (which would be derived from multiplying your "mass density" * volume * gravity), which does impart a static load on parts. Unless you can enter in part weight in an alternate form, I would still make sure you get these "mass density" values correct.

 

 

MikeT

[Alex Khenkin]

There's no such thing as "mass density". Density is defined as the MASS OF A UNIT VOLUME. There's no weight involved - a given material does not change its density when moved from Earth to the Moon because neither the mass nor the volume change.

- Alex

[Steve Cole]

Ahh yes.... "Mass Density" is a term that gets shoved down your throat quite a bit these days when you go through Engineering school in the US (not to say that you didn't, but as I still live off pasta and top-romin I would consider myself somewhat fresh out of school).  The units are basically "lbm (pounds-mass)/volume (in^3, ft.^3...whatever units you are in) OR "sluggs/volume".  I gueess there has been so much confusion over "pounds" (i.e. "Is that force or mass?") that they like to make sure you know what you are doing.  Anyway, the confusion should all be set to rest when the U.S. comes to their sences and goes metric.... until then, you get to keep your standard AND metric tool set.

 

To make thing weirder.... lbm=lbf (mass pounds=force pounds) on earth...so lbm is not a true synonym for kg.

 

kg = N/g <--"1 kg= metric mass which will move 1*m/s^2 when a 1N force is applied)

lbm = lbf/(normalized g) <--"1 lbm = standard mass which will move 32.2 in/s^2 when a 1 lbf is applied"

sluggs = lbf/g <--"1 slugg = standard mass which will move 1 in/s^2 when a 1 lbf is applied"

 

So... 1 slugg = 32.2 lmb....so you better know which units the program is expecting or you are going to have another mars polar lander on your hands .

 

2nd thing:  I don't know what you are analyzing, but Mass Density can come into play on a static analysis in the form of weight (which would be derived from multiplying your "mass density" * volume * gravity), which does impart a static load on parts.  Unless you can enter in part weight in an alternate form, I would still make sure you get these "mass density" values correct.

MikeT

11069[/snapback]

 

Thanks again Mike.

First of all concerning the US going metric anytime soon I wouldn't hold my breath, I've been hearing that rumor since 1975.

 

You are also right about the Mass Density being a factor in the stress calculation. I would be concerned if i were analysing a large structure or a piece of heavy machinery. My primary use of Algor is in analyzing weak points in molded plastic parts. The material densities relative to loads on these parts is insignificant. If I use the Materials from the Algor library the Mass Density is there anyhow. If I had to enter the material spec for something not in the library like a polyurethane do you know how I would calculate the Mass Density given to materials density?

SteveC

[Mike Twining]

Cause I am lazy and don't like to think much, I take the "density" of a material (hopefully in kg out of a book somewhere) and thow it in to my trusty Ti-85 and let the computer do the work for me (there are several unit conversions in the calculator). The only thing you need to be carefull with is determining if your program is looking for lbm or sluggs (when working in standard unity). That I can't help you with, but I would suggest the old "F1" in Algor...

 

...now, cause I am somewhat bored, here is a little bit o' knowledge from the Ti-85:

 

1kg = 2.2 lbm

1kg = .0685 sluggs

1 slugg = 32.174 lbm

 

Enjoy.

 

MikeT

[mmccall]

HA... here we go again...

 

Nobody should feel left out... we had a real train wreck here on several designs over this same issue.. we even got the Physics Profs involved.. and you know how a PHD can make you feel like an idiot... !!

 

Things get even stranger when working in Mechanica... some people eliminate the problems by making sure they apply gravity to the session...

 

We even do monthly classes where all the contractors have a meeting covering ANALYSIS problems... very important around here...

 

I will get the notes from that particular session.. and type them in.. maybe they will help us all... since the PE's sign off on all the work.. I guess we can trust them... HA..

Edited September 7, 2005 by mmccall