Rubber dunnage bags are designed to give cargo the highest level of protection while still being lightweight, long-lasting and easy to handle. They’re tough enough for repeated use, simple to repair when needed, and they’re built in Britain to meet strict performance standards, including the American Bureau of Shipping Test Report SO 309140, which confirms that pressure loss won’t exceed 30% over 21 days.
One of their most useful design features is the corner-positioned valve. Because it sits right at the edge, the bag can be fully inserted into the gap before inflation, ensuring you get the maximum stabilising force exactly where it’s needed.
Safe Use and Recommended Equipment
Dunnage bags are straightforward to operate, but they must be used correctly. Anyone unfamiliar with the instructions shouldn’t attempt inflation, and they should never be filled from an unattended air line. The risk of over-pressurising the bag is serious and could cause severe injury.
The safest option is to use our inflation tool, which features a built-in gauge and trigger mechanism. The gauge should be observed during inflation, and it is still essential to monitor, until the indicator changes to a green colour, which indicates a pressure of 5-6psi within the bag.
The bags also use a purpose-designed Checkmate corner valve; a reliable, low-cost, easy-to-maintain and fully compatible with both pressure-maintenance systems and handheld filling pistols.
The Mechanics of Air Dunnage Bags
Although the engineering behind dunnage bags can sound mathematical, the way they work is actually very simple. Their strength and performance are shaped by the size of the gap, the pressure in the bag, and the amount of contact they have with the cargo.
- How the Bag Handles Pressure
When a bag is inflated, the air inside pushes outward on the rubber. The wider the gap, the rounder the bag becomes and the more tension that is created in the material. If this tension gets too high, the bag risks failing.
This is why dunnage bags are more secure in narrower gaps and why the recommended maximum gap is 30cm. In a smaller space, the bag can press firmly against the cargo without overstretching.
To put it simply, a narrow gap allows the bag to stay flatter, causing less stress and making it safer. A wide gap causes the bag to bulge more, causing higher stress and a greater risk.
- How Much ‘Holding Force’ Do You Get?
The more surface area the bag has in contact with the cargo, the more force it can apply to keep the load still. As the gap gets bigger, the bag loses contact area, so the available bracing force drops.
Even if the bag is strong, a large, open gap simply doesn’t allow it to grip the cargo as effectively.
- Why the Bag Must Sit Fully Inside the Gap
If a dunnage bag sticks out of the gap even slightly, two problems happen at once. First, the exposed part becomes rounder than the section inside the gap, which puts extra stress on the rubber. Second, that exposed section behaves like an unnecessary air reservoir. When the cargo shifts and the gap closes, air gets pushed back into the smaller space. Because the bag’s reinforcement has a little elasticity, this creates a “spring” effect, reducing the bag’s ability to resist movement.
In practice, a partially inserted bag can slowly work its way out, lose pressure where it’s needed, and eventually pop out completely. In this position, it’s essentially ineffective.
- Why Pressure Changes During a Voyage
Even a perfectly inflated bag won’t hold the same pressure forever. Several natural factors cause pressure to drop:
- Minor air leakage: Rubber is slightly permeable, and older bags lose air faster.
- Cargo shifting: As cargo settles, unbagged gaps close and bagged gaps widen, reducing pressure in the bag.
- Atmospheric pressure changes: A simple gauge only measures pressure relative to outside air; weather changes affect readings.
- Temperature changes: Heat increases pressure, cold reduces it, often dramatically.
- Material relaxation: After inflation, the bag stretches slightly over the first hour, causing a small initial pressure drop.
All these factors interact, especially on long journeys, and together they can significantly reduce pressure if the bags aren’t monitored or topped up.
