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Old 21st September 2017, 07:55 AM   #241
TheGnome
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Ah yes. So they are metric. But if you can't unscrew a nut, you don't care if it's of metric or imperial size. You just have to notice that it's left threaded.

Happened to me a few times with certain makes or models of TV sets where the picture tube was attached to the case with left threaded bolts.

Damn if I know why they did this.
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Old 21st September 2017, 09:00 AM   #242
Speedskater
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Bicycles have one pedal reverse threaded. With a normal thread, the pedal would loosen itself over time.
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Old 21st September 2017, 09:00 AM   #243
CORed
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Originally Posted by TheGnome View Post
Ah yes. So they are metric. But if you can't unscrew a nut, you don't care if it's of metric or imperial size. You just have to notice that it's left threaded.

Happened to me a few times with certain makes or models of TV sets where the picture tube was attached to the case with left threaded bolts.

Damn if I know why they did this.
I have no idea why they would use a left-hand thread for picture tubes. Some older cars (mostly Chrysler products, IIRC) used to use left-hand threads on the wheel lug nuts on the left side if the car, I believe so that inertia would not loosen them when braking. It could make trying to change out a flat difficult if you didn't know they had to be turned the "wrong" way. I think this was abandoned because, as long as the nuts are properly tightened, they won't come loose anyway, and it probably cost less to use the same studs and nuts on all the wheels.

ETA: I've also run into left-hand threads attaching fan to water pump, where the direction of rotation was such that the water pump could unscrew itself from the fan with a right hand thread.

Last edited by CORed; 21st September 2017 at 09:03 AM.
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Old 21st September 2017, 09:02 AM   #244
bruto
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Originally Posted by TheGnome View Post
Ah yes. So they are metric. But if you can't unscrew a nut, you don't care if it's of metric or imperial size. You just have to notice that it's left threaded.

Happened to me a few times with certain makes or models of TV sets where the picture tube was attached to the case with left threaded bolts.

Damn if I know why they did this.
To keep people like you from getting the picture tube out and cutting yourself?

Left threads make a certain sense in certain applications, among them bicycle bottom brackets and pedals. The pedal threads (also several different types) are always, or at least almost always, left threaded on the left, but bottom bracket threads, despite the sense of it, are not if they are French or Italian.

Some things, like the left side wheel lugs of automobiles, should be, and used to be in many cases, but rarely are nowadays.
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Old 21st September 2017, 09:05 AM   #245
ceptimus
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British Association screw threads are some of the weirdest around. B.A. fasteners are still used by model makers as they often look more like full-size fasteners scaled down to model scale than any of the alternatives.

From Wikipedia
Quote:
They are unusual in that they were probably the most "scientific" design of screw, starting with 0BA at 6.0mm diameter and 1.0mm pitch and progressing in a geometric sequence where each larger number was 0.9 times the pitch of the last size. They then spoiled this by rounding to 2 significant figures in metric and then converting to inches and rounding to the thousandth of an inch. This anticipated worldwide metrication by about a century. The design was first proposed by the British Association in 1884[1][2] with a thread angle and depth based on the Swiss Thury thread,[3] it was adopted by the Association in 1903.
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Old 23rd September 2017, 12:59 PM   #246
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Originally Posted by BillC View Post
This statement, while sort of true, is missing some of the details. The current across the heart is driven by the voltage, and although the impedance is highly non-linear, the higher voltage, in principle the higher the current. While a shock from a Taser might carry 50kV, it is non-lethal, while a shock from a 50kV power line is almost certain to be. What makes the difference is the impedance of the internal circuit and how long that voltage is maintained. A Taser has a high internal impedance and delivers its voltage in short duration pulses. The power line has a very low impedance, and will sustain its power flow until external protection kicks in and trips the circuit, by which time the shock is likely lethal.
Mmm, impedance is not all. Of course, a high impedance source can deliver little current, even with a high voltage, but it is also a question of capacity, or very often, capacitance. A taser delivers quite a bit of current, but with very short duration. The amount of energy supplied is also important.

Hans
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Old 23rd September 2017, 03:38 PM   #247
psionl0
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Originally Posted by MRC_Hans View Post
Mmm, impedance is not all. Of course, a high impedance source can deliver little current, even with a high voltage, but it is also a question of capacity, or very often, capacitance. A taser delivers quite a bit of current, but with very short duration. The amount of energy supplied is also important.
I think BillC was referring to the impedance of the source - not the body. Capacitance is also part of the body's impedance anyway.
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