April 18, 2019
In this entry of Tips From the Canopy, ....
In the last installment, we ventured down the road of Mechanical Advantage and discussed “Working Smarter (safer) and harder” in relation to the job site.
For this entry, we’ll dive into Mechanical Advantage and which gear we need. As with anything, if we can increase efficiency and make our jobs easier, while at the same time increasing safety, it is certainly worth considering.
Definition of mechanical advantage
the advantage gained by the use of a mechanism in transmitting force
specifically : the ratio of the force that performs the useful work of a machine to the force that is applied to the machine
Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system.
In other words, we can move or lift an object with less work.
It’s highly recommended in this industry to have a mechanical advantage readily available on the job site. It comes in handy in many situations and is something that should be utilized more than it is in the industry! On the plus side, you probably already have some of the gear needed to set up a Mechanical Advantage kit.
Before looking specifically at your gear, let’s talk about why you would invest in the gear to begin with. The predominant use would be when felling, but there are additional uses, such as lifting or pre-tensioning rigging lines. Most scenarios we’re faced with daily are residential settings — where obstacles are abound, and to be honest, that’s part of what makes this job fun and interesting! And it is one of the reasons you’re hired in the first place. It’s the challenge of getting the tree safely to the ground without causing destruction or harm, and doing it with total control. This is where mechanical advantage can be an extremely useful tool.
A mechanical advantage system can be simple, but it proves very effective. For a small investment, you can put a kit together, and you likely have some — if not all — the required components already. A couple of double sheave pulleys, carabiners, prusiks, and a 50’ – 75’ piece of rope and you are in business.
On average, we’re capable of pulling roughly between 50% to 60% of our body weight. So, a 200 lb person can likely pull between 100 to 120 pounds. If we introduce a simple 2:1 mechanical advantage, then the same person can pull 200 to 240 pounds.
Even this amount of weight can be enough to ensure the tree is going where you intended. That said, it is important to understand that there is no replacement for proper cutting and felling techniques. A Mechanical Advantage (MA) set up can only do so much so be aware that it is vitally important that your cuts be executed properly.
There are several pulley options available and they will all work. Just be aware of the interaction and compatibility with ropes and other pieces of hardware. Sharp edges do not play well with ropes especially when under tension. Several good options are available from popular and reputable manufactures such as Rock Exotica, Camp, Petzl, and CMI. The Camp Dryad Pro pulley with double sheaves and becket sports a compact design which is beneficial when hauling the system around and setting up. The Omni blocks from Rock Exotica are a great option as they do not require the user to remove the carabiner from the top attachment point to open the side plate. This can make installing the rope less cumbersome during set up. The Omni Blocks are available with double sheaves in two different sizes. The Petzl Twin Pulley is another option and works well and has been around for some time and proven its reliability in the field.
Another item that isn’t a necessity — but is strongly encouraged and likely already in the gear bag – is a portawrap or other basal friction device. When using a mechanical advantage system, the length of rope in the system will quickly run out. If we are using a 50' section of rope in a 5:1 system, we really only have 10' of pulling length. Integrating a portawrap into the system provides you with a way to capture all of your progress and securely lock down the host line when the blocks touch each other, and you can no longer pull any rope through the system. Once the host line is locked and secured, you can move the top block in the MA system back up the host line and reset the system. You can then begin to pull again if necessary.
We can also utilize a MA system to lift branches and limbs. One common scenario is using a MA system to lift a limb over a house or other structure. Lifting the limb during the rigging allows us to remove the limb and keep it at the same height after removal so we do not run the risk of hitting any structure (for example, the house). Once the limb is suspended we are able to manipulate the limb up and over the house to a safe drop zone. The GRCS is a great example of a tool built for the job and excels in operations like this. We can also use our 5:1 system with pulleys to lift, however the lifting capacity is much less when compared to the GRCS. If you find yourself in situations where a MA system is needed regularly, consider investing in the GRCS. I have never known anyone to have buyer’s remorse after adding a GRCS to their kit. But if the expense of a GRCS is not something you can take on, definitely consider adding a 5:1 MA system to your kit as it will quickly pay for itself.
Next time, we’ll look closer at the physics of mechanical advantage. If you’ve been on the fence about adding this type of set-up to your kit, ask others to see what they use and what benefits they’ve found from this type of system.
Climb safe. Cut safe.
Review by(Posted on 12/11/2019)
it would have been nice to see pictures of MA lifting branches, thanks
Review by(Posted on 12/11/2019)
Either aluminum or steel would be fine as long as they are strong enough. Using the numbers given in the article and assuming a 200-pound Crewmember develops 60% of his body weight in pull (120 pounds), then if you are setting up even a 5:1 system, that Crewmember using the MA system is only going to develop 600 pounds of pull in the system.
For safety's sake, I would never use a carabiner for personal support that had been used in this type of MA system. If, for example, you are using the system to pull over a large tree and the tree starts to come over but then spins on the stump so that the MA system develops slack and is then shock-loaded as the tree falls to the side and is "caught" by the MA system, it is nearly impossible for you to precisely measure the load applied. Perhaps it was not overloaded, but perhaps it was. When one of our Crews asks for a snap to use with a pull rope, the snap gets a quick coat of red paint to mark it as a snap that is never to be used again for personal support.
Two other subtle points about using an MA system...
The multiplied force is multiplied much further by getting your pull point high up in the tree you're trying to pull over; the higher the anchor point the greater the leverage to pull the tree over.
Anchoring the MA system far to the side of the tree you're cutting will make it much more effective. If you anchor close to the tree then you're pulling mostly DOWN instead of sideways, not to mention that you put yourself at greater risk of being struck by the falling tree or something that the tree hits and propels toward you.