This is an unusual post. It was prompted by a website I spotted on Hacker News, devoted to conveyer belts.
As a teenager I briefly worked at the end of a delivery conveyor belt for a supermarket. A conveyer belt was unusual for a supermarket, and the supermarket itself was unusual: It's located over a major highway. It's the Star Market (since purchased by Shaw's) over the Mass Pike/I90 outside of Boston:
Why does a supermarket straddle six lanes of an interstate? The story dates back to the 1950s and early 1960s when the state and city of Boston were undertaking a series of massive infrastructure and redevelopment efforts, including the construction of the Massachusetts Turnpike. Whole blocks of homes and businesses in Boston and Newton were razed to make way for the toll road. The Star Market in Newtonville was one of the businesses that was demolished. However, the Turnpike Authority was simultaneously developing the idea of "air rights" over the new highway, and the first two development projects that benefited were Star Market in Newtonville as well as the Prudential Center in Boston. Both opened in 1963. Additional air rights were granted for other Pike parcels, including the hotel in Newton Corner (opened in the late 60s) as well as more buildings in downtown Boston.
Getting back to the conveyer belt: The supermarket had an unusual layout. Customers had to take an escalator to the second floor to do their shopping. After checking out, their groceries were loaded into carts. There were 2-3 paper grocery bags per cart, each of which was numbered from 001 to about 500. The customers were given plastic cards with the corresponding numbers for their carts.
The baggers swung the carts onto the top level of a double-decker conveyer belt. It went down to the first floor (street level) and into a long, basement like room with a conveyer belt and a road paralleling it. Customers would drive their cars into this long room, pop the trunk, and hand me their cards. I would match up the bags, and place them in the trunk. The empty carts were placed on the bottom level of the conveyer belt, to be brought back to the Muzak-filled main level of the supermarket.
The room was filled with fumes and noise from the waiting cars and the Mass Pike tunnel that was next to it. The incessant rattling and squeaking of thousands of metal rollers on the conveyer belts were irritating -- upstairs where the customers were it was an actual belt, which was quiet, but down where we were it was those damn rollers, which were like 1950s-era metal roller skate wheels. We were paid $3.85/hour (minimum wage at the time). But the things that worried us from day to day was the cry of "mix" (human error, wrong bags placed in wrong car) or a spill.
Here's what happened with the spills. As the carts came from the 2f to 1f, they went through a series of turns, including at least one 90 degree turn and a full 180 at the bottom of a decline. This spot was where most of the spills took place. It was apparently unavoidable, owing to the layout of the store, the needs of the customers to get their cars loaded quickly, and the location of the slopes on the belt, the road and loading area. The nature of groceries (heavy/light loads, multiple packaging sizes, etc.) and the technology used at the time made it hard to find an easy fix to the problem. Watermelons rolling around the bottom of the carts were the worst.
I don't have any profound observations about this, other than spillage is a consideration for people who design and manage conveyer belts, and that the cost can be made manageable for both small and large systems. And these belts can be engineered to last years or decades. The belt that we used in that market was in use for more than 20 years by the time I started working there in the 1980s, and it (or a similar system, using the same route) is still in use today, some 50 years after it was installed.