Your torque converter's runnin' low on torque ...
But don't be downhearted, I can fix it for you, sonny.
It won't take too long, it'll just take money.

("Talkin Song Repair Blues," by D. Linde sung by Alan Jackson, 2005)

 

TORQUE TALK

Brilliant PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order recently learned about Shaft Couplings, the mechanical device that transmits the rotary power which enables the "Driving Shaft" to rotate the "Driven Shaft."

 

PTOA Segment #185 also mentioned that ...

• The selection of the optimal Shaft Coupling depends primarily upon Torque and ...
• For a given and maintained steady horsepower, the rotational speed of the Shaft increases as the Torque of the Shaft decreases ... and vice versa.

 

The nearby graphic illustrates the second bullet point.

The scale of the "Percentage of Revolution Speed" shown on the X axis (aka "% of Rotational Speed") increases from left to right.

The scale of the  "Percentage of Torque" (shown on the Y axis) increases from bottom to top.

The blue line showing graphed Torque linearly decreases as the speed in RPMs of the Shaft increases. The significance of this observation is explained later in this PTOA Segment.

Right now it is evident that this thing called "Torque" sounds mighty important!

So what is meant by Torque?

This PTOA Segment #186 focusses on the meaning of Torque because understanding the concept of Torque is a prerequisite to understanding the form and function of Primary Movers/Drivers, Couplings, and even the bolting patterns used for flanged connections which will be covered in a future PTOA Segment.

Hey ... bolt patterns should be used for proper tightening of wheel lug nuts, too!

It is Torque that provides the originating rotational movement that makes it possible for the Driving Shaft to thence transfer rotational power to the Driven Shaft.

This PTOA Segment also reviews how Gears are used in Rotating Equipment to step down (or step up!) RPMs and thus impact the force of Torque output from a Shaft ... which up to this point has been an unexplained duty of the Shaft Coupling.

 

TORQUE DEFINED

PTOA Readers and Students learned way back in PTOA Segment #61 that Humankind has assigned conceptual names to observed relationships which explain how the Universe works.

For example, Humankind assigned the concept name of "Velocity" and "Speed" to the observed relationship of "distance" divided by "time."

This relationship between Velocity (v), distance (d), and time (t) is shown in the nearby graphic.

The definition/mathematical expression for "Velocity" is now so familiar to everybody that nobody even remembers it was made up from Humankind's observations about the Universal relationship of "distance" and "time." Heck, the great apes don't stress about it!

Well ... Torque is just another definition/mathematical relationship that was determined after Humankind observed how the Universe works.

Torque is the fancy name given to a Force that generates rotational movement which causes something to rotate around and around. That's all it is!

Torque is different than the simple "linear" or straight line Force that can push a block left or right or upward or downward.

The "linear, straight line" Force is the same type of Force that was spread out over a defined Area which defined the concept of Pressure way back in PTOA Segments #142 and #143.

Torque is different because it causes rotational movement, instead of straight line movement. And rotational movement can be used to create rotational power that does useful work ... like building up the PV Pressure in fluids!

For example, the nut in the nearby graphic is being "torqued"  ... or twisted ... by a wrench.

This twisting generates a different kind of rotating Torque Force (which nerdy types call "A Moment").

Hey, that is so weird and interesting that it is worth repeating!

When a lever (like a wrench) is pivoted about a fixed center by a Force that is applied some distance away from that center ...

A Torque Force (called "a Moment" by nerds) is generated and rotates around and around in a circular path from that fixed center.

The blue circular arrow at the top of the nut illustrates the circular, clockwise movement of the Torque.The directional, winding Force which creates the Torque is drawn on the hand.

There are a few more things to notice about Torque:

Torque has a direction!

Your Mentor can tell the nut in the graphic is being tightened because the Force applied by the hand on the lever of the wrench is clockwise and ...

"Rightsy-Tightsy, Leftsy-Loosey!"

Ahem! Translation, please!

In the USA, a clockwise movement applied to a fitting like a nut will result in tightening the fitting 99% of the time (notable exception: propane cylinders attached to backyard barbecues).

Although it's admittedly hard to tell from the graphic featuring the wrench, the Torque Force is directionally downward  ...which would tighten the nut.

When the nut needs to be loosened ...

the hand in the graphic would move the  lever of the wrench anti-clockwise ...

thus the Force applied to the lever formed by the wrench will likewise become anti-clockwise ...

and the magnitude of the Torque will directionally point upward as shown in the nearby graphic ...

thus loosening the nut.

Since Torque has a direction and a magnitude ...

Torque generates thrust ... so bearings will always be necessary wherever Torque is generated in industrial Rotating Equipment. Like ... duh!

So what will be the magnitude of the Torque that is generated when a Force is applied to a pivoting lever?

Ta-dah! It's time to introduce the definition/mathematical expression for Torque!

As the below graphic illustrates:

The magnitude of the Torque depends upon the amount of Force that moves the lever and ...

The length (L) between the pivot point and the point where the Force is applied ot the lever (which is where the hand is grasping the wrench in the previous graphic). 

Hey! That must mean when L=0, no Torque will be generated.

Righteeo!

Thanks to David Waldorf for the nearby photo that shows "Superman" about to be choked in a revolving door.

If he wants to live, "Superman" should not be applying a Force away from the pivot point at the center of the door!

Even better ... "Superman" should push on the opposite door in the wedge because the clockwise-generated Torque will free him from the debacle.

 

TORQUE AND SHAFT COUPLINGS

Applying what was just learned about Torque clarifies why Torque is of primary importance when sizing Shaft Couplings.

After the Prime Mover/Driver generates Torque, its rotating Shaft transfers rotary power to the coupled Shaft of the Driven Rotating Equipment and its attached "Load."

The magnitude of the Torque generated by the Primary Mover/Driver and the efficiency of transferring that rotary power in the Shaft Coupling will determine how much work the Rotating Equipment "Load" can perform to fulfill its intended job of building up the PV Pressure in fluids.

 

Two important things to stop and ponder at this point:

The "Load" of the Driven Rotating Equipment is a generic phrase that is used to describe any piece of Process Industry Rotating Equipment that needs rotary motion to build up the PV Pressure in Liquids and Gases (e.g. any pump or compressor type).

Without the generation of Torque, it would be impossible to dynamically build up the PV Pressure in any Fluid!

 

 

THE RELATIONSHIP BETWEEN TORQUE AND HP AND RPM

Torque, Power (aka Horsepower=HP), and rotational speed (RPM) have another important relationship:

HP = (Torque X RPM) / 5252

The "divided by 5252" part of the equation makes the units of this definition of hp consistent with the definition of hp introduced in PTOA Segment #168.

Recall that Power/Horsepower is the amount of work that a power-producing machine can do over a specified period of time. Examples of power-producing machines are Motors and Engines.

The above definition/mathematical expression for HP explains that ...

• A Primary Mover/Driver produces (Horse) Power by rotating a Shaft at a given RPM which exerts a given amount of Torque on a Load. And...

• If HP is constant ... that must mean any upward change in RPM must cause a downward change in Torque, and vice versa!

 

Heck! That's what the blue line in the nearby graphic shows!

 

SNEAK PEEK

Hey, Your Mentor can tell we are making headway!

Fred "got it" that it was a human hand which provided the Force on the wrench-lever and that's what generated the Torque Force that tightened the nut.

But now he wants to know ...

What provides the Force within the Prime Mover/Driver that turns its Shaft and begins the generation of Torque, RPMs, and thus the ability to generate power (HP) in the first place?

Aha!  That question will be answered within the upcoming PTOA Prime Mover/Driver Segments!

 

ADJUSTING RPMS ... AND THUS TORQUE ...WITH GEARS 

In the future PTOA Co-Generation Focus Study, PTOA Readers and Students will learn about Gas Turbines (GTs).

 

GTs are a type of Prime Mover/Driver that can transfer rotational power not only to its Load (which is often an Electricity Generator) but also to the Shafts of auxiliary equipment (e.g. lube oil pumps, hydraulic pumps, alternative/backup fuel pumps, etc.).

The optimal rotational speed of the Load and auxiliary equipment are often different than the optimal design rotational speed and Torque Force output that the GT Prime Mover/Driver generates.

Therefore, Gears are mounted between the GT and whatever rotating equipment must be driven.

Accessory Gear Boxes may even be required.

Gears can decrease the RPMs of the Driven Rotating Equipment.

Gear teeth are also in the Flexible Coupling that was introduced in PTOA Segment #185. The Gear teeth transmit Torque, yet can still slide a bit to keep the Flexible Coupling within alignment tolerance.

 

 

HOW REDUCTION GEARS WORK

The (difficult to read) nearby graphic shows two intermeshed Spur Gears with differing diameters.

The Spur Gear that would be threaded onto the Driving Shaft has 18 teeth and rotates clockwise. The Driving Shaft is also referred to as the "Input" on the graphic.

The Spur Gear that would be threaded onto the Driven Shaft has 24 teeth and rotates anticlockwise. The Driven Shaft is also referred to as the "Output" on the graphic.

 

Determining Gear Ratio

The Gear Ratio between two meshed Gears states how many times the Driven Shaft will rotate compared to (usually) one rotation of the Driving Shaft.

The Gear Ratio is  determined by dividing the number of teeth on the Driven Shaft (aka Output) by the number of teeth on the Driving Shaft (aka Input).

The Gear Ratio of the Driven/Driving Shaft is 24/18 = 1.33 to 1. This Gear Ratio means the Driving Shaft will rotate 1.33 times while the Driven Shaft rotates just once.

Taking a different point of view ...

The Driven Shaft (aka Output) would turn just 0.75 times when the Driving Shaft (aka Input ) turned 1 full rotation (because 18/24 = 0.75).

So, if the Driving Shaft is spinning at 1000 RPM, the Driven Shaft would spin at just 750 rpm.

 

A 1:1 Gear Ratio is used when there is a desire to keep the RPMs between intersecting Shafts constant but change the direction of rotation and Torque.

TYPICAL GEAR PROBLEMS

Since Gears are used to transfer rotary power, PTOA Readers and Students should intuitively deduce that Gear failure would cause inefficient operations if not catastrophic equipment failure.

The nearby photo exhibits  Gears that are no longer useful with respect to changing RPMs and therefore rotational power.

The root cause of most Gear problems is improper alignment of one type or another.

Sometimes the problem is bad alignment while the equipment is resting cold and sometimes the problem is an unstable structure supporting the Gear.

Because Gears have metal to metal surfaces sliding by each other, they can be damaged by the causes of Wear that were featured in the  4 Part PTOA Tribology Focus Study, specifically covered in PTOA Segment #178.

Pitting of Gears is caused by frequency stresses which start below the metal surface and progress until a cone shaped bit of metal falls out. The root cause of Scoring, Scuffing, and Galling of Gears is insufficient lubrication.

Normal everyday Wear is caused by abrasive particles in the lubricant.

The root cause of actual Gear teeth breakage is harder to identify. Teeth breakage might begin with Scoring that causes pits to grow until a fatigue crack breaks off a tooth of the Gear.

 

COMMON GEAR TYPES FOUND IN INDUSTRY

The Spur Gear shown in the nearby graphic is the most common type of Gear and is usually used with two parallel Shafts. The straight teeth of the smaller Driving Spur Gear are meshed with the straight teeth of the larger Driven Spur Gear. Spur Gears are used for Gear Reductions in the range of 1:1 to 6:1. The Gear Ratio for the Spur Gears in the nearby graphic appears to be 40/14 = 2.85. Spur Gears are noisy and create vibration.

The Helical Gear has teeth that are cut at an angle and can be used with parallel or skewed Shafts. When two mating teeth of the Helical Gear mesh together, the contact starts at one end of the tooth and gradually continues up the length of the teeth. Thus, their contact is much quieter than Spur Gears which explains why they are commonly used in automobile transmissions. Helical Gears are used for Gear Reductions in the range of 3:2 to 10:1.

Bevel Gears are used on intersecting Shafts so they change the direction of rotation.  Their Gear Ratio range is 3.2 to 5.1. Hand drills use a Bevel Gears to change the vertical rotation of the handle to the horizontal rotation of the drill. Bevel Gears are also found in helicopter engines, automobile transmissions, and mechanical garage doors.

Rack and Pinion Gears convert rotational movement into linear movement. The "Rack" is the long, flat member of the Gear and the "Pinion" is the Gear wheel.  "Rack and Pinion Steering" made driving automobiles much easier. The steering wheel rotates the Pinion which rolls on the Rack. As the Pinion rotates on the Rack, it slides the Rack either to the right or left, depending upon which way the steering wheel is turned.

 

Worm Gears are used when large Gear Reductions in the range of 20:1 ... and even in excess of 300:1 are required!

The Worm can turn the Gear but the Gear cannot turn the Worm. Worm Gears are often used in conveyor systems.

 

TAKE HOME MESSAGES: Torque is a twisting Force that creates rotational motion. Torque is generated when a pivoting lever has a Force applied to it, which causes a Torque Force (or Moment) to be created around the radius of the pivot point. 

Torque has a direction and a magnitude. The direction of the rotating Torque Force depends upon if the lever is pushed clockwise or anti-clockwise. The  magnitude of Torque depends upon the length of the lever from the radius to where the Force is applied and the amount of Force that is applied to push the lever clockwise or anti-clockwise.

Because Torque has a magnitude and direction, wherever Torque is generated thrust bearings will be needed.

Torque is generated within the Prime Mover/Driver and creates the rotary power that is transferred from the Driver Shaft to the Driven Shaft within the Shaft Coupling. 

Torque is one of the factors that creates Horsepower ... which can be used to perform mechanical work. The other factor that creates Horsepower is the RPMs of the Driver's rotating Shaft. Without Torque it would not be possible to perform the work to dynamically build up the PV Pressure in fluids.

At a given steady HP, if Torque increases then RPMs decrease and vice versa. 

Sometimes the optimal RPMs of the Load is less than the RPMs of the Driver Shaft. In this case Reduction Gears are needed to reduce the RPMS of the Driven Shaft and also the Shafts of Auxiliary Rotating Equipment.

The Gear Ratio relates how many times the Driven Shaft will rotate compared to (usually) one rotation of the Driving Shaft. Gear Ratio is determined by dividing the number of teeth of the Driven Shaft(aka Output) by the number of teeth of the Driving Shaft (aka Input).

Beside reducing RPMs, Gears can also be used to change the direction of rotation by proper selection of Gear type and Shaft orientation.

Gears have metal surfaces that mesh with other metal surfaces and must be lubricated lest they be worn down.

Common types of Gears are Spur, Helical, Bevel, Rack and Pinion, and Worm.

 

©2018 PTOA Segment 0186
PTOA Process Variable Pressure Focus Study Area
PTOA PV Pressure Rotating Equipment Focus Study
PTOA PV Pressure Prime Mover/Driver of Rotating Equipment Extension Study

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