Re: Re: Paris Tramway T3a on youtube
  Dudley Horscroft

There is rather little in the PCC book on 'jerk'. To start with, "Hirshfeld's chief assistant Emil H. Piron, released his
preliminary findings to Westinghouse and General Electric. He asked them to design and market a control package that would
accelerate a PCC car at a jerk-free rate of 4.75 mphps, something that had not been possible before." Note, 4.75 mphps is 2.1234
m/s^2. I have heard, but cannot recall the source, that in the experiments, a jerk rate of 0.8 mphps^2 was considered the limiting
factor - anything more was uncomfortable to passengers, anything less could be considered 'jerk-free'. (page 144)

"One field shunt and a single fixed resistance were temporarily inserted as soon as the line switch closed to minimize starting
torque. These same two circuit elements were inserted when power was released to reduce the similar jerk that would otherwise be
apparent." (page 146)

"During the 1960s, traffic congestion in several cities still using the cars became so intense that a 42-mph car was no longer
desirable. Both Philadelphia and El Paso found that a 25-mph car was acceptable and thus removed the field shunting.
Unfortunately, because the shunts were also used to cushion acceleration, it made the cars somewhat jerkier." (page 160)

The speed-time/distance curve shown on p 152 for a car with empty weight of 36 000 lbs, passenger load of 6000 lbs, and a rotary
inertia of 5 600 lbs, gear ratio of 7.17 to 1 and wheel diameter of 25", average voltage of 550 V and set acceleration rate of 3.5
mphps, train resistance at "Davis plus 6 lb/ton, and on level tangent track, shows that acceleration gradually increases from zero
to about 5 mph (about 2s). It then is approximately constant till about 10 mph (about 4 s), decreasing thereafter till about 40 mph
(70 s). The increasing rate at the start is presumably when the 'anti-jerk' shunts were being cut out.

Also on page 152 is a listing of balancing speeds for early PCC cars. Figures are given for passenger loads of 40 and 93 - 40
presumably for most of the passengers being seated, and 93 for sardine conditions. Noteworthy is the balancing speed of 17.0 mph
for 93 passengers up a gradient of 12%. So much for trams not being able to climb hills of more than 8% - or 4% as I seem to recall
on something recently posted! Can't recall what and have looked but it must have been in the last few days.

"PCC book means "PCC - the car that fought back" Interurbans Special 64, Carlson and Schneider III, 1980.

Regards

Dudley Horscroft
----- Original Message -----
From: "Prescott" lenkaprescott@...>
To: "TramsDownUnder" tramsdownunder@...>
Sent: Friday, March 22, 2019 9:04 AM
Subject: Re: [TramsDownUnder] Re: Paris Tramway T3a on youtube


>I must say that I've never experienced a jerky start or stop on a tram.

> Look forward to the day when I find one! It must be limited to a few cases

> where either the manufacturer's or the operator's techs haven't done their

> job properly.

>

> Tony P

>

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