thats a decent pulley if you're forced to do speedlining, having the becket on it for attaching another line for lowering down the line slower. I've got a few of the tandems, some with bearings some with bushings.
The necessity for speedlining rarely arises IMO. The last one I did was last year, large elm (3ft trunk) growing at nearly a 45 degree angle 4 feet from a neighbors fence so the whole top of the tree was over his yard and he wasn't the least bit cooperative. We had plenty of room and a large black locust to use as an anchor so we set it up with the GRCS to tension the line and the peices rode on those tandem pulleys, carabiners and slings. Zipped all the limbs right to the chipper.
Its fun, but in most cases IMO not practical due to the time involved setting up, and hauling back up the hardware every few rounds. We got paid by the hour on that one under a subcontract anyway tho so w/e.
In case you didn't already know, its better to hang the rigging line over a strong branch near the union or even better through a sling/block combo, and secure the standing part of the rope around the main trunk. The force created by the peices on the line will put more downward pressure on the tree/crotch rather than a pulling pressure, which imo is safer, less likely to fail.
