Can I add all torques in a shaft and use it as the Tmax?

[Ballena Joseph]

I have a problem on determining the diameter of a shaft. And I am required to find its maximum twisting moment/maximum torque (Tmax). Is it possible to add all the torques in different points of a shaft to be able to use it as the maximum torque (Tmax)?

 

[CWatters]

With care... If the shaft is running at a constant angular velocity (eg not accelerating) the net torque will be zero.

 

[Dr.D]

If there is power entering and leaving the shaft at several different points (such as a drive pulley on a line shaft powering several machines), you need to find the torque in each segment (between entry and exit locations) to determine which is the largest.

 

[Ballena Joseph]

If there is power entering and leaving the shaft at several different points (such as a drive pulley on a line shaft powering several machines), you need to find the torque in each segment (between entry and exit locations) to determine which is the largest.

Yes! That's what my Technical Report is all about. I need to design a line shaft with 3 pulleys and 1 gear. I am required to determine the diameter of the line shaft (just one diameter of the whole shaft). What if I already known the maximum torque in the line shaft? Should I ignore the other torques in each section of the shaft?

 

[CWatters]

Perhaps this should be in the homework section. I've seen similar problems posted on the forum. Your textbooks or course material should also help you solve this.

 

[CWatters]

As I recall there is an acceptable factor for the amount of twist that a section of the shaft can have so things like the distances between the applied torques matter. I recommend posting the problem word for word with diagrams in the homework section using the template provided in that section of the forum.

 

[berkeman]

Perhaps this should be in the homework section.

Thread moved to the schoolwork forums.

 

[Dr.D]

There are two factors that combine to determine the required diameter for a shaft: (1) the torque in the section, and (2) the bending moment in the section. You cannot properly design the shaft without a knowledge of both of these.

 

[CWatters]

Perhaps..

https://www.physicsforums.com/index.php?threads/926793/

 

[Dr.D]

Perhaps..

The source quoted at that previous PF article seems to indicate that the shaft can be properly designed by considering torsion alone. I think that is in error.

Any shaft with transverse loads (such as belts or gearing) that is not continuously supported (i.e. bearing support at every section) must be designed for combined bending and torsion as both are present at every location along the length. My old machine design book (Mechanical Engineering Design, Shigley, 3rd ed) discusses this topic at length in Ch11. There are many different situations involving combined static and dynamic stresses in combination, but it is simply not safe to look at torsion alone.

 

[Ballena Joseph]

The source quoted at that previous PF article seems to indicate that the shaft can be properly designed by considering torsion alone. I think that is in error.

Any shaft with transverse loads (such as belts or gearing) that is not continuously supported (i.e. bearing support at every section) must be designed for combined bending and torsion as both are present at every location along the length. My old machine design book (Mechanical Engineering Design, Shigley, 3rd ed) discusses this topic at length in Ch11. There are many different situations involving combined static and dynamic stresses in combination, but it is simply not safe to look at torsion alone.

Design: Line Shaft
In a machine shop, power is supplied from a 75 hp, 1125 rpm electric motor through flat belt drive to a counter shaft with 24" Cast Iron pulley weighing 150 lbs. 80% of this power is taken by two identical lathe machines, No. 1 and No. 2, operating at 600 rpm, 8 ft below through another flat belt driver with 18" diameter pulley and weighs 100 lbs. each on the counter shaft. 10% of a motor power goes to the blower through gear drive. The remaining power is assumed as losses due to friction, slippage and other mechanical losses.

a) Make Shear and Moment diagram.
b) Compute the diameter of the shaft by code (ASME Code) if material is to be machined from AISI 1045 as rolled.
c) Compute the diameter by Octahedral Shear Theory.
d) Same as item (c), but by Maximum Shear Theory.
e) Determine the maximum torsional deflection of the shaft in item (b) and compare it with values from code (ASME Code). Is it safe?
f) Compute the lateral deflection on shafting.
g) Compute for the critical speed of the shaft.
h) Specify flat leather belt drive by (1) ALBA, (2) by Stress Analysis.
i) If flat belt is changed with V-belt drive, specify the drive.
j) Same as item (i), but roller chain drive.
k) Specify gear drive if compressor (blower) runs at 900 rpm.

I'm stucked at item (b). I already get what you said about torsion and bending moment, but I don't know what formula should I use after taking the maximum torsional and bending moment in a particular segment of the shaft, since I am only required to compute the diameter of the shaft (only one diameter for the whole line shaft).

 

[Dr.D]

I don't know what ASME says about such things, so I cannot address that. Is it in your textbook?

You need to apply a failure theory, such as the maximum shear stress theory, and test this at every point along the length of the most highly stressed section. I'd program this, and test it in small steps from one station to the next because it is usually not possible to eyeball the location of the worst condition. You may also want to do some on-line research for the Westinghouse formula, the Sines approach, and the Kececioglu approach. I suggest getting a copy of the machine design texts by Shigley or the one by Spotts if your own text is inadequate. This is a great problem, and you will learn a lot if you carry it through.

 

[Ballena Joseph]

I don't know what ASME says about such things, so I cannot address that. Is it in your textbook?

You need to apply a failure theory, such as the maximum shear stress theory, and test this at every point along the length of the most highly stressed section. I'd program this, and test it in small steps from one station to the next because it is usually not possible to eyeball the location of the worst condition. You may also want to do some on-line research for the Westinghouse formula, the Sines approach, and the Kececioglu approach. I suggest getting a copy of the machine design texts by Shigley or the one by Spotts if your own text is inadequate. This is a great problem, and you will learn a lot if you carry it through.

Yes, it is in my textbook (Design of Machine Elements by V. M. Faires). And I have a copy of Shigley's (9th Edition) pdf.

 

[Dr.D]

Ah, V.M. Faires! He was on the faculty of North Carolina State and his book was used at Texas A&M. Some of my Aggie friends have told me that a common quiz question was to ask students what the V.M. stood for. My copy of Faires' text is the 3rd ed., date 1955. I looked there, and sure enough, there was a reference to an ASME Code for Shafting, but no information about what the Code says. I hope your copy has more information.

 

[Ballena Joseph]

Ah, V.M. Faires! He was on the faculty of North Carolina State and his book was used at Texas A&M. Some of my Aggie friends have told me that a common quiz question was to ask students what the V.M. stood for. My copy of Faires' text is the 3rd ed., date 1955. I looked there, and sure enough, there was a reference to an ASME Code for Shafting, but no information about what the Code says. I hope your copy has more information.

My copy of his book is the 4th Edition, date 1962 or 1965? I'm not sure.

 

[CWatters]

A quick google suggests that the ASME methods says something like..

"..the permissible shear stress for shaft without keyways is taken as 30% of the yield strength in tension (Syt), or 18% of the ultimate tensile strength of material (Sut), whichever is lower."

 

[Ballena Joseph]

A quick google suggests that the ASME methods says something like..

"..the permissible shear stress for shaft without keyways is taken as 30% of the yield strength in tension (Syt), or 18% of the ultimate tensile strength of material (Sut), whichever is lower."

Thanks. But I already searched for it. And it is also in my textbook (Design of Machine Elements by V. M. Faires), that tells it is based on Maximum Shear Theory.