Gross versus Net
Several years ago, a customer had ordered a new screw for
their thin sheet process. They requested a throughput rate
of 1600 lb/hr.
The new screw was designed so that 90% of horsepower
available was consumed, which resulted in a maximum
throughput of approximately 1800 lb/hr at full screw speed.
Shortly after the customer had received and installed the
new screw, they called and complained that they were only
getting 1200 lb/hr when they had requested 1600 lb/hr. We
spent several phone calls trying to diagnose the problem and
nothing seem to change the throughput rate of the screw, it
stayed at the 1200 lb/hr.
It was then decided that I would have to travel to the
customer’s Chicago facility and resolve the problem. After I
had received much grief from the company’s General
Manager upon my arrival, we went out on the factory floor.
The first thing that I told the customer that we needed to do
was to shut the line down so that we could do a rate check
directly from the lips of the sheet die. So we did our rate
checks at 25, 50, 75 and 100 percent screw speeds. And
the rates were within 2% of the theoretical calculated design
rates. I was totally baffled as to why we got the expected
rates at the die but the GM was complaining that they were
only getting approximately 75% of the theoretical rate.
So it was decided to string the sheetline up to see if that
gave us any indication as to why they were not getting the
full throughput of the screw that they expected.
After stringing up the line, the operator pulled the slitter
knives into position. Immediately I knew what was the
problem. The knives were set to slit the 48” sheet to 36” wide
and they had 6” of trim on each side of the sheet. Well, 36”
divided by 48” equals 75% and 75% time 1600 lb/hr equates
to 1200 lb/hr!!!
The throughput rates that the GM was seeing in his
production report every day was the NET weight excluding
the 12” of trim. I had to explain to him that we determine the
GROSS throughput rate as what comes out of the die, not
what he winds into his rolls.
Believe It or Not!