What is the difference between left-hand thread and right-hand thread?

Answer 1

The difference between left- and right-hand thread is that one is turned in a given direction to tighten the fastener, and the other is turned in the opposite direction to tighten the fastener. Let's look a bit more closely.

Most fasteners with threads are turned clockwise to tighten them. (This is true for a nut, and for a bolt, you'd be looking at the head with the threads away from you.) These are right-hand threaded fasteners. These fasteners are turned counter-clockwise to loosen them, as you might have guessed. With left-hand fasteners just the opposite is true when tightening them or loosening them.

We make left hand fasteners for special purposes. We'd have to have a reason to make "backwards" threads on a fastener, wouldn't we? We do, and most applications where we see left-hand threaded fasteners involve attaching rotating parts. A lawn mower blade is a good example, and by looking at the physics, it will become obvious.

A lot of blades on lawn mowers attach to the shaft of the motor with a nut or a bolt. You know the machine we're talking about. The motor shaft points straight down, and the blade is a "cutter bar" with the ends sharpened. Anyway, if the motor on the machine turns clockwise (looking at the motor from the bottom where the blade is attached), using a fastener that turns clockwise to tighten it (a right-hand threaded fastener) sets the stage for a problem. Roll up your sleeves and look at the physics.

When the motor starts, it "torques" the shaft clockwise. The blade and the fastener that hold the blade are likewise torqued clockwise. But consider mean old inertia. Inertia is the name we give to the phenomenon associated with objects that have mass (like the fastener that holds the blade on). If a body is at rest, it wants to remain at rest. If it's in motion, it wants to remain in motion. Force is necessary to change the motion of a mass because of inertia. That said, let's focus on the bolt that holds the blade on the motor.

If the bolt on the motor shaft of a lawn mower turns clockwise and a right-hand threaded bolt holds the blade on, the inertia of the bolt when it's at rest makes it "resist" the torque of the motor, which is clockwise. The effect on the bolt is that the bolt's inertia makes it want to turn counter-clockwise. And counter-clockwise is the direction we'd turn the fastener to take it off. If the motor is started time and time again, it applies a little torque in the "wrong" direction to the bolt that holds the blade on. If the bolt ever becomes a bit loose, it can easily be spun off, which will cause the blade to detach. If the blade comes off while the machine is running at speed, that blade becomes a lethal projectile.

Left-hand threads are applied in a number of applications, and most involve rotating parts. It costs more to make left-hand threaded fasteners because the volume in which they are produced is lower. The economy of scale makes the right-hand threaded fasteners cheaper, and they would be a manufacturers choice if not for the engineering considerations associated with the rotating parts of a machine. Left-hand threaded fasteners are made for a specific application where right-hand threaded fasteners would be a fool's choice.

Answer 2

With standard screws, which are right-threaded screws, they are turned to the right to tighten, and to the left to loosen. With left-threaded screws, they are turned to the left to tighten, and to the right to loosen, thus the saying, "Lefty - loosey, righty - tighty". The left-threaded screws are mainly used on something mechanical that rotates to the right, to keep them from tightening up as the machine rotates. Another use for left-threaded fittings are butane/propane bottles. They are used as a safety feature on them so you can't screw a common fitting into the bottle.

Answer 3

in right hand threads th threads slope up vertically right n vice-vera for left hand..........