Ratchet Screwdriver Without Ratchet With Overrunning Clutch

Ratchet is a mechanism that permits the tool to apply turning force only in one direction while moving freely in the opposite. There is another mechanism that does the same- overrunning clutch. I created a 3D-printed version of the clutch and integrated it with a screwdriver.

Overrunning clutch does not require strong small parts so it's possible to create it on a 3D printer furthermore, it does not crack, unlike ratchet.

The main idea of the clutch is to wedge a shaft in one direction and release it in another by metal cylinders placed into special slots. It is widely used on bicycles.

Supplies

You can use different parts that can be found in your workshop: hex bits, a tube, nails, screws, washers, and nuts. Also, you need to print some parts on your 3d printer.

Step 1: Prepare Wedging Cylinders From Nails

Overrunning clutch is a bearing with cylinders so you can use any metal cylinder. I made it from nails.

Step 2: Prepare STL Files and Plastic Parts

All parts for the screwdriver are in the Fusion360 model file attached to the post. Download the file and open it in Fusion360. There are 4 bodies for each part.

This is one of the most important steps. You have to find the best dimensions of the overrunning clutch for your tube and nails. The clutch, the tube, and the nails have to work together as a single mechanism. That is why I would recommend printing 5-10 small cases of the clutch with different parameters and testing which one is the best for your purposes.

There are 6 main parameters in the Fusion360 model for tests:

Shaft Diameter - the tube diameter
Stopper Diameter- the nail diameter
Stopper Spacer- the space between the released and blocked position in the stopper slots. It defines the tightness rotation of the tube in the clutch.
Stopper Height- the height of the clutch, for the tests you can print a 20 mm version to save plastic.
Shaft Inner Diameter - it is used for printing screwdriver shank fixer
Screw Shank Diameter - it is used for printing screwdriver shank fixer

Measure the dimensions of your parts and play with these parameters in +- 0.1, 0.2, 0.3 mm. I printed at least 10 cases of the clutch and tested how they work. Try how they work in both directions (blocking and releasing). After that print the "big" version of the clutch 40-100 mm in height.

In order to change parameters open the model in Fusion360, menu Modify -> Change Parameters. After that make overrunning clutch visible in browser view and export STL files in menu File -> Export -> STL Files (*.stl).

When the best params are found, fill the following params

Press Washer Diameter - diameter of washers that are used as press bearing (see following)
Press Washer Height

You can play with the rest parameters, and watch what is changed on the 3D view after editing parameters.

When you find the best parameters for the clutch select the Cover1, Cove2, ShankFixer in Fusion360 browser and export them. As a result, you have STL files to be printed. I use PLA plastic and 1.2 - 3mm for walls, top, and bottom in CURA.

At the end of the step, you have the following 3D printed parts

Overrunning clutch
Top/Bottom Covers
Shank Fixer 2 pieces

ScrewDriverParts.f3d
Download View in 3D

Step 3: Prepare the Tube

The tube is a rotating part of the clutch. I would recommend using a steel tube 10-20 mm in diameter (diameter must be bigger than shank diameter) and 1.5 - 3 mm in thickness. The surface of the tube should be a little bit rough, so use sand if it's polished.

The tube is used to rotate a screw shank that is why you need to put a special 3D printed fixer into the tube with some glue.

Step 4: Prepare Clutch Covers

Step 5: Assembly

Use M3-M5 screws with nuts to assemble the clutch with the covers and place a screw shank into the clutch. Sometimes you need to use a special 3D printed plastic spacer for keeping the shank on one side of the clutch.