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+# Ploopy BTU Mod
+
+The roller bearings used in the Ploopy Trackball are nice, but with the standard ball they might not feel as smooth or as low-friction as you'd like. The prompt for this mod was "what if we replaced them with ball transfer units?"
+
+It turns out - good things happen!
+
+Ball transfer units (or BTUs) are omnidirectional bearings, as opposed to the single-direction roller bearings the Ploopy Trackball uses by default. This means you're never rolling the ball in the "wrong direction" for one of them.
+
+This mod is a new 3D-printable top piece for the Ploopy which contains holes for ball transfer units instead of roller bearings.
+
+Designer: [George Bryant](https://github.com/gbrnt) with help from [/u/d4baller](https://reddit.com/user/d4baller)
+
+## Versions
+### v3
+This version was designed in Rhino and so isn't really editable for most people. It works well though.
+
+### v4
+This version was redesigned in FreeCAD and the various dimensions are now editable in the FreeCAD file.
+
+Currently untested! It might collide with something internally, particularly if you've soldered in a programming header for the microcontroller.
+
+## Buying the BTUs
+The particular BTU this mod is designed for is the Bosch-Rexroth R053010810, or KU-B8-OFK. A set of three is about 20-30 USD, depending on where in the world you are and where you buy them from. Here are some sources we've found so far:
+
+* [Bosch-Rexroth](https://www.boschrexroth.com/en/xc/products/product-groups/linear-motion-technology/ball-transfer-units-and-tolerance-rings/ball-transfer-units/r0530-with-sheet-steel-housing/r0530-1-with-sheet-steel-housing)
+    * UK (and worldwide?)
+    * Requires you to be a company (have a VAT number)
+* [Livhaven](https://store.livhaven.com/r053010810-bosch-rexroth-ball-transfer-unit)
+    * US-based
+    * Doesn't require you to be a company, but overseas shipping is expensive
+* [L-System](https://l-system.nu/webshop/linear-technology/ball-transfer-units-and-tolerance-rings/ball-transfer-units/type-r0530/ball-transfer-unit-ku-b8-ofk/)
+    * Sweden-based
+    * Allows you to select "private customer" but still has mandatory field for VAT number
+* [Spareparts](https://www.sp-spareparts.com/en/p/r053010810-bosch-rexroth)
+    * Ireland-based? Ships to UK, and maybe elsewhere.
+    * Does not require you to be a company.
+* [Rodivago](https://rodavigo.net/es/p/bola-transportadora-con-carter-de-metal-laminado-ref-053010810/557053010810)
+    * Spain-based
+    * May require being a company.
+