I have a 3500 lb winch and want to add a block so I can double it up so what do I need?

Question by aaron j: I have a 3500 lb winch and want to add a block so I can double it up so what do I need?
I didn’t know if I should get a block that will hold 6,000-7,000 lb or if one for 3,000 lb will hold.

Best answer:

Answer by alfredo
You need a snatch block rated at least twice the rating of your winch. It will take all the force of the mechanical advantage, relieving the force required on the winch. The problem with that is answering the question of specifically how much force is applied to the block, because it depends on the angle of recovery, as you may not always be pulling at the same angle if you are able to use trees or other vehicles to assist in the operation. In a recovery, you can’t always pull directly toward the direction of “discovery” as I call it. I have “discovered” a lot of places from which I needed to be recovered, if you see what I mean. In short, your snatch block should be rated at 7000 to 10000 lbs to be safe. Here is the long explanation:

Snatch blocks are one of the most mis-understood and misused items in the winching field, and can cause enormous harm and death if not properly rigged. I have witnessed a broken, whistling cable and block abruptly breaking lose, taking out all the glass in a truck, narrowly missing several people watching. A few feet in the wrong direction and it would certainly have killed one of us, so don’t take the risk lightly. Remember, if you overload a cable/snatch block, and the block parts from the anchor, you have a 10 or more pound missile flying at great speed, which can kill instantly. The most important thing to remember about snatch block winching, as used in the typical 4wd sceneario, is that there is tremendous force applied to the block/anchor even when using a single-block system for a simple change of line direction.

> = vehicle
O = block
# = anchor
- |= line

1. >———–O|
No mechanical advantage, 1 pound of pull required for each pound of lift/tension.

2. >—————-O|

Mechanical advantage = 2/1 1 pound of pull results in 2 pounds of lift/tension.
Now for the IMPORTANT part; The amount of force applied to the *block* is dependant upon the angle between the incoming line (to winch) and the anchor line (to tree)! The total load on a block can be as much as *2 times* the load applied. The maximum force on the block comes when the angle of the two lines is 0 degrees, or are parallel to each other as shown in #2 above. The actual load varies with the angle between the legs.

Here is the table from the Crosby Group catalog:

Angle is the measured angle between the legs of the line as they pass over the block sheave. Factor is the multiplier for the applied line load to calculate the block load.

Angle Factor
0 2
10 1.99
20 1.97
30 1.93
40 1.87
50 1.81
60 1.73
70 1.64
80 1.53
90 1.41
100 1.29
110 1.15
120 1
130 0.84
135 0.76
140 0.68
150 0.52
160 0.35
170 0.17
180 0

In #1 above, the angle is 90 degrees, so the factor is 1.41. So a 10,000 pound load will place 14,100 pounds of force on the block/anchor.

And in #2 above, if the bitter end of the line is *not hooked to the winched vehicle* but is hooked to a tree or rock next to it, for each pound of winch force applied, the block will be under *twice* the load, while the line strain will be equal to the line pull. So if you apply a full 12,000 pounds of pull, you are putting 24,000 pounds of stress on the block/anchor. If, however, the bitter end *is* attached to the winched vehicle, the block becomes a “traveling block”, even though it is attached to a solid object, because the vehicle itself is actually doing the “traveling”, and the load on the *line* is halved, since you are using two lines to “support” the load. In this case, the mechanical advantage is 2/1, so you get 2 pounds pull for each pound of winch effort. Now, since you have two lines splitting the load, let’s say it’s 10,000 lbs. (you’re *really* stuck), the line load is halved to 5000 lbs per line, but the *block* load is still 10,000 lbs because of the parallel line angle factor of 2. As you can see, while the load on the winch is cut in half, as is the line speed for winding, the load on the block is *not*, and is equal to the total line load.

This is why using a properly rated and carefully maintained block is of the utmost importance when winching. In most RV winching situations, “double-lining” is the best method to reduce stress and strain on the lines and winch motor, at the expense of speed, but may be the *worst* thing you can do from the safety or block standpoint, if your block is not adaquately rated or is in poor condition or if you don’t know the strength of the anchor or the stall-pull of your winch. The block is being subjected to twice the force of any of the other components, and consequently the winch operator may never know he is exceeding the capacity of the block until it parts and takes his head off. This is especially true when using very powerful winches.

Imagine the stress on a block (and it’s anchor

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