As a mom of a young child, it is completely understandable that you would want to be as safe as possible. Nobody wants to use an unsafe product and put your child at risk of injury or death. So making sure that you have a safe baby carrier is very important.
There have been a lot of babywearing groups and stores that sell baby products in recent years recommending “pull testing” as a way to test that the carriers are safe. The Baby Carrier Industry Alliance has also recently come out with a strong stance against this kind of testing as a potential risk to your child.
So, where does that leave us? That is two conflicting points of view on the same topic. So it is time to look at some examples of what is really happening with baby carriers and how to test them.
The reason that the Baby Carrier Industry Alliance is against this form of pull testing is that it puts extreme pressures on a carrier and can ultimately weaken the carrier by fatigue. An related example of this is how the National Highway Transportation Safety Board recommends replacing all car seats after car accidents even fairly minor ones. The reason here is the same, extreme forces can weaken the product because of stress and fatigue.
Now lets just consider for a moment a baby carrier that is designed to support 40 pounds. In general there are 4 straps, 2 hips and 2 shoulders. This would work out to say that each strap is designed to hold an average of just 10 pounds. (Everything is simplified/rounded for this article) For reference, a gallon of milk is over 7 pounds. Now when you pull test a carrier it is very easy to pull with more then 10 pounds of force. Lets say that a pull test is done with 20 pounds of force. Multiply that by the 4 straps and the carrier was basically pull tested for 80 pounds when it was built for just 40.
Now the real extreme cases happen when someone grabs a strap and the body of a carrier and just pulls not in the direction of the forces when being worn. When looking at that same strap that is designed for 10 pounds of force, and the strap is 4 inches wide, that works out to being able to support about 2.5 pounds per inch. If a carrier were to be pull tested in a manner that puts all of the force on ½ of an inch at the edge of the strap, such as not pulling straight, that could be 20 pounds of force on ½ inch or 40 pounds per inch. That is a lot more then the 2.5 pounds it was designed to support. Multiply that 40 pounds per inch times the 4 inch wide strap and the 4 straps on the carrier and that pull test was the equivalent of testing it for a baby weighing 640 pounds. That is why some carriers will fail this type of pull test.
The more important thing here is the potential danger. When products are sewn, there are a number of types of stitches used such as straight stitch or chain stitch, but all stitches get weaker and not stronger when one stitch in the chain is broken. A very heavy thread such as a polyester T-92 thread may have a tensile strength of 7.2 pounds. When a strap is sewn to a carrier it may be attached with 8 stitches per inch for example. This would mean the thread goes up and down 8 times in one inch for a total of 16 supporting strands, multiplied by the 4 inch strap and the 7.2 pound tensile strength and the stitching on that strap should be able to hold about 460 pounds of force if all the stitches are sharing the load equally. If a pull test is performed and the tension is not applied squarely the way the carrier was designed, all of the force my be placed on 1 or 2 threads that can each hold only 7.2 pounds. Once a single thread breaks, the rest of the stitching becomes weaker and can start to come undone.
So inspecting a carrier for broken threads is very important, while “pull testing” a carrier can be dangerous. It is advised that you inspect your carrier closely where ever straps meet or webbing joins with straps. Look for broken threads. If it is just the end of a line of stitching, it is fine. Look from broken stitches in the middle of a line of stitches. This would indicate that the carrier may be on its way to failure and should be replaced.
As an alternative to pull testing your carrier, the only safe thing to do is purchase carriers that have been tested by independent labs. For example the European testing standard BS EN 13209-2:2005. This testing standard for baby carriers is tested by independent third party labs. As part of this testing there is strength testing.
In the strength testing portion of this standard, the carrier is fastened to a torso, loaded with the maximum weight the carrier is designed for and then bounced 4 inches high at 50,000 times over 7 hours. This is pretty serious testing. They use the max weight and basically drop the carrier 4 inches, twice per second all day long.
They then examine the carrier for things like damage and slippage of the straps. This is a very fair test and far more rigorous then any pull testing of a carrier, and far safer to purchase a product that has 3rd party testing for safety But the safety standard testing does not stop with the 50,000 drop test. They also test for things like grip tests, bite tests and strangulation hazards among many other things.
The best thing to do when purchasing a carrier is to only purchase a carrier that has passed some form of third party testing. It is the safest thing to do and should give you peace of mind in knowing the product you are using is safe.