Thrust Loads

Hi to all,
I am a Mechatronics student, in which we are studying about thrust loads on bearings. In or text book, it gives the definition “A thrust load acts in a direction parallel to the shaft axis and opposite to the direction of force transmission”. I know this forum is for PLCs in which I have a great interest in; but in Mechatronics we study electrical, electronics (including PLCs), mechanical, pneumatics, and hydraulics. With that all said; my question is, do they mean the force pushing on the thrust load such as the weight of a robotic arm would be on a thrust bearing (thrust load) pushing up against the weight pushing down on it? I just want to make sure I understand what their definition means. Thank you all for help. Have a great day.

Thrust loads are normally loads applied axially to a spinning shaft.

a robotic arm (that mimics a human arm-remember that in PLC's we have robots that move every which way...)) would incur very little thust loading. Now, if you stood on the side of the robot, and attempted to push its arm into its shoulder (pushing sideways on the arm, where it attaches to the robo-shoulder)- this would incur some thrust loading- this is a bad example though. try wikipedia:

http://en.wikipedia.org/wiki/Thrust_bearing

Thrust bearings are used to oppose/control thrust loading.

-John

Johmster's explanation is absolutely correct, but this might help you visualize it better.

Take a common machine, say your sump pump sitting with the shaft vertical, sitting on a couple of bricks in the bottom of your basement's crock. The pump shaft has to be restrained in all three directions. The motor has a bearing on the top end and the bottom end that keep the shaft from moving horizontally in either direction. However, the weight of the motor and pump impeller would make th pump shaft try to drop down and the impeller to rub on the face of the pump casing. The weight creates a thrust force along the shaft. The motor therefore also has a bearing to take the thrust load and hold the shaft in position vertically.

Thank you all for your help. The problem I am having is what they mean by opposite to the direction of force transmission

The problem I am having is what they mean by opposite to the direction of force transmission

Click to expand...

Consider Tom Jenkins' example- the weight of the motor "pushes" down on the pump impeller.

Its newton's third law that says: " The mutual forces of action and reaction between two bodies are equal, opposite and collinear." (from wikipedia again...)
-Our two bodies are the motor and the output shaft
The motor has mass/weight and is always pushing down on something (gravity)... its "force transmission" is downward.
The output shaft- since it is being pushed down by the motor, it has to either 'push back' or accelerate downward. Since your sump is a finite size, the output shaft with thrust bearing pushes back. Its "force transmission" is upward.

In this case, the 'action' is that of the motor pushing down on the shaft. the 3rd law means that something (in this case, a thrust bearing) must push upward on the motor ("...opposite and collinear")




My favorite example is of the 'propeller shaft' on a ship. The ship's engines are not meant to be pushed, so the propeller shaft (the shaft that connects the screws to the engine) has a thrust bearing near the middle - when the engine turns the screw, the screw pushes the water backard, which pushes the shaft forward. Since we want the ship to go forward (not just the shaft) we put a large thrust bearing that 'anchors' the shaft to the hull of the ship. The ship is really being pushed by the thrust bearing. This creates a potentially interesting dynamic when, on some ships, the engine is in the forward part of the hull, the screw is in back, and the thrust bearing is in teh middle - the ship is being pushed forward from it's middle...not the most stable setup. (in reality - most ship/boat thrust bearings are located very far aft, and there is no stability problem)

-John

They are talking about Newton's 3rd Law of Motion: "For every action, there is an equal and opposite reaction."

I don't know, guys. I can see why Maintenance Man is confused. I don't like the definition that is given.

When I think of "direction of force transmission" for a rotating shaft I think of tangential force off the shaft. It is the force that the shaft is twisting against. I would say that a thrust force is orthoganal to the direction of force transmission.

I like the way Wikpedia states it. A trust force is an axial force on the rotating element.

Keith

Thank you all for your help, I really appreciate it. I guess a easy example would be a nut and bolt? If a person is turning a bolt in a nut, the force transmisson would be the bolt moving in one direction as it turns in to the nut, and the thrust load would be the nut moving in the opposite direction ; or do I have the nut a bolt mixed up where the thrust load would be the bolt being turned and the force transmission would be the nut?

Think of a propeller on a ship. The propeller is on a rotating shaft which is held in place (radially) by bearings. As the propeller turns, water is pushed away from the ship, to the rear. Now that whole opposite reaction thing. Since the propeller pushed the water to the rear, the water is pushing the propeller (and the shaft it is attached to) toward the front of the ship (axially). This force is thrust. A thrust bearing is attached to the hull of the ship and a collar is fixed to the shaft. The collar pushes against the thrust bearing which in turn pushes the ship. I've stood next to some VERY large thrust bearings during my time in the Navy.

Shawn

Thank you all for the help. The example Shawn 75 gave me helped me understand. Hope you all have a great day.