What is the relationship between stress and load for a 19mm shaft?

[learnphysics10]

I have been given this question by my teacher as part of a quiz and I'm confused on how to start.
1. Calculate the maximum working tensile load that a 19mm shaft is able to take with a safety factor of 3. Material UTS 470 Mpa
2. Stress = load/area

When it states 'area' what is this the are of? The 19 mm shaft? If so, how do I possibly work it out without any other measurements of the length.

Please feel free to correct me if I'm wrong with any of my attempts.

3. -Stress is the maximum tensile load
-UTS 470 Mpa means ultimate tensile strength 470 mega pascals?
-The load is 470 Mpa
-stress= 470 Mpa/?

Any help would be greatly appreciated :)

 

[bp_psy]

You should include the info you have about the safety factor and notice you want to compute a load not a stress.

When it states 'area' what is this the are of? The 19 mm shaft? If so, how do I possibly work it out without any other measurements of the length.

I suppose that by 19mm your prof means the diameter or radius of the cross section of the shaft which you should be able to compute using what you are given.

 

[Chestermiller]

You don't need to know the length. If the UTS is 470MPa, and the factor of safety is 3, what is the maximum tensile stress that you are allowed to impose of the shaft? If the shaft diameter is 19mm, what is the shaft area? At the maximum allowed tensile stress, what is the tensile force on the shaft?

 

[learnphysics10]

What does the 'safety factor of 3' do in terms of this equation? So the formula stress= load/area is not needed to calculate maximum working tensile load? The area of the shaft is 9.5^2 x π = 283.52

 

[haruspex]

What does the 'safety factor of 3' do in terms of this equation?

A safety factor of 3 just means that you should arrange the actual stress not to exceed one third of the theoretical limit stress.

 

[Chestermiller]

What does the 'safety factor of 3' do in terms of this equation? So the formula stress= load/area is not needed to calculate maximum working tensile load? The area of the shaft is 9.5^2 x π = 283.52

No. It is used. It's just used in the reverse order than what you've been thinking. First you determine the maximum working stress, then you determine the maximum working load. A factor of safety of 3 means that the maximum working stress is equal to the ultimate stress divided by 3.

 

[learnphysics10]

then you determine the maximum working load.

How do you go about determining the maximum working load? Is it 470 Mpa?

 

[Chestermiller]

How do you go about determining the maximum working load? Is it 470 Mpa?

It's 470 divided by the safety factor.

 

[SteamKing]

It's 470 divided by the safety factor.

The maximum working stress is 470 MPa divided by the safety factor.

I believe the poster was inquiring about the maximum working load, which is a different animal. It's what's required to answer the OP.

 

[Chestermiller]

The maximum working stress is 470 MPa divided by the safety factor.

I believe the poster was inquiring about the maximum working load, which is a different animal. It's what's required to answer the OP.

Yipes. You're right. He finally got me confused.

 

[learnphysics10]

So the formula for stress = load/area is not needed for this?

 

[SteamKing]

Well, let's analyze this. You are looking for the maximum load. You know the maximum working stress. You know the size of the shaft. How do we relate stress to load for a shaft?