It's called an impact test hammer or resiliometer. It is more commonly used in testing the hardness of concrete, but has an application in web offset printing. It is used by paper companies and can be used by your press room. It is inexpensive and can pay for itself many times over.
How It Works
By pressing a small cylinder into the roll, a hardness reading can be obtained by the vibration produced of the spring impact. Don't worry though, you really don't have to understand the science behind it. What the device will give you is a measurement of the hardness of any particular spot on a roll (see video). Generally, there will be a reading somewhere around 45 newton meters on regular newsprint.
What's more important than the reading itself is the difference in reading form side to side. If you measure a reading of 35 one one side of the roll and then get a reading on the other side of 45, then you have a roll that was wound with uneven tension. This is a paper defect that can have various consequences on the press. The tool is important because this is difficult to determine by feel. Even paper companies will hit the roll with a stick to hear the difference. However having a measured reading will will give you ammunition in a claim against the paper company since this instrument is recognized by them as a reliable test.
How Much Is It?
We purchased ours new for about $250 on ebay. It's easy to spend thousands on these devices, but it is really not that necessary for this application. A simple impact test hammer will do.
Why It's Necessary
Here's why every press room needs one. We have a flying paster. It works by splicing full speed onto an expiring roll. Occasionally the splice will miss. It's not that often, but it happens. Often the cause was that the expiring roll was slightly baggy on one side. This of course is a paper defect that can be claimed with proper evidence. However, there is no way to prove this in an acceptable way to paper companies. With an impact test hammer, if a reading of the hardness is provided, this is an acceptable way to make a paper claim. This most often happens with rolls from a position at the end of the jumbo at the paper mill.
How It Can Pay For Itself Quickly
With paper companies compensating for press downtime (due to their own manufacturing defects) as much as $1000/hr, this device is definitely worth the investment. If a splice fails because it was not as tightly wound on one side, indicate the impact test hammer reading for the roll. If there was a significant difference, you have an acceptable form of evidence for a claim.
It can also help to troubleshoot other web press problems that may happen with tension, slur, or folder problems. You only need to see the benefit of it once.
It might sound like I'm trying to sell these things, but I'm not. You'll have to do your own searching if you want to buy one. The version we bought was a no-name brand from China. You may find something better. All I can say is, it is definitely worth the small investment.
Wikipedia - Impact Test Hammer
Web Press Troubleshooting Guide