On Falling Cameras

An overhead camera rope snapped and fell onto the track during the Coca Cola 600 Sunday evening.  FOX Sports released the following statement:

“Everyone at FOX Sports is relieved and thankful to know that the injuries to fans caused when CAMCAT malfunctioned Sunday at Charlotte Motor Speedway were minor, and those who received hospital treatment were released last night. As stated previously, we regret that the race was affected, and we apologize to the racers whose cars were damaged, to everyone at CMS, NASCAR, and NASCAR fans, especially those who were hurt. At this time, we still do not have a cause for what happened, but a full investigation is underway, and use of the camera is suspended indefinitely.
“The rope is made of Dyneema, an ultra-strong synthetic that has the same approximate strength of a steel wire with the same diameter, and is less than a year old. According to the company, it had been factory-tested by the manufacturer and its breaking strength is certified before shipment. It was also inspected by CAMCAT upon receipt last June. The rope was certified to have a breaking strength of over 9,300 pounds. The force exerted during last night’s race was less than 900 pounds

Dyneema is a member of a family of polymers called ultra-high-molecular-weight polyethylene (UHMWPE).  Ethylene’s high school picture is shown at right:  Ethylene is also known as  C₂H₄ – because it has two carbons and four hydrogen atoms.  The parenthesis with the ‘n’ means that the unit is repeated ‘n’ times.  The words “ultra-high-molecular weight” can be translated simply as “lotsa”.  There may be a couple of million units in your typical UHMWPE fiber.  Lots of fibers are combined and you end up with something that is extremely strong, but lightweight.

Dyneema has some additional positive qualities relative to other types of UHMWPE.  Dyneema has extremely low friction (comparable to Teflon), which means it is good for allowing things to slide on it smoothly.  It also is resistant to abrasion, which means it should be durable.  UHWMPE has a strength-to-weight ratio of 8- 15 times more than steel (see this video).  This should give you an idea why the falling cable was able to damage race cars.  The reports I saw during the Television kept saying that the rope was “nylon”.   Nylon can certainly cause damage when it hits something at 180 mph, but something equivalent to a steel cable is definitely going to be dangerous at high speed.

The CAMCAT system (shown at left) is an aerial camera system used by many sports broadcasters.   The low friction, abrasion resistance of the Dyneema allows a camera to travel up to 80 mph over lengths of 700 yards.  FOX has suspended their use of the technology until they figure out what happened.

Dyneema does have a couple of issues.  Almost every polymer is sensitive to ultraviolet radiation – Materials like Nomex and Kevlar degrade when exposed to UV radiation from the Sun.  Dyneema builds up a UV-opaque sheath during its initial time in the Sun, so it is much more resistant to UV degradation than other similar polymers.  A Dyneema fiber should have  no problem supporting the 900 lbs of the camera – it ought to be able to support more than ten times that weight… if the fiber is ideal.

Dyneema doesn’t have very good high-temperature properties.  Although its melting point is upward of 250 degrees F, when you hang something heavy on a cable and leave it for a long time, the fiber is subject to creep.  “Creep” is the technical word for something stretching because you keep pulling on it.  You could also say “sag”, I guess.  I’d be curious about how much weight was left hanging on the line.  I’d also ask about whether there were any splices and if the break was near the splices.

I’ve never seen a piece of television equipment affect a race like this before.