プリミティブを組み合わせて歯車を作るエディタースクリプト。Hingeジョイントなんかに値が入っているけど、値に根拠はないよ！

GearMaker.cs

using UnityEngine;
using UnityEditor;
using System.Collections;

public class GearMaker : EditorWindow
{
	int toothNum = 0;

	[MenuItem("GameObject/Gear")]
	public static void makeGear()
	{
		GearMaker.GetWindow(typeof(GearMaker), false, "Gear");

		return;
	}

	void OnGUI()
	{
		Event e = Event.current;

		bool keyPress = false;
		switch (e.type)
		{
			case EventType.keyDown:
				switch (e.keyCode)
				{
					case KeyCode.Return:
						keyPress = true;
						break;
				}
				break;
		}

		int num = this.toothNum;
		EditorGUILayout.BeginHorizontal();
		{
			num = EditorGUILayout.IntField("歯数", num);

			if (num < 0)
				num = this.toothNum = 0;
			else
				this.toothNum = num;

		} EditorGUILayout.EndHorizontal();

		if(num > 4)
		{
			if (GUILayout.Button("作成") | keyPress)
			{
				this.makeGear(num);

				this.Close();
			}
		}

		return;
	}

	void makeGear(int num)
	{
		float angle = 360.0f / (float)num;

		float rad = (360.0f / (float)(num * 2)) * Mathf.Deg2Rad;
		float leng = 0.5f / Mathf.Tan(rad / 2.0f);
		float pos = leng / 2.0f;

		GameObject parent = new GameObject("Gear" + num);
		Transform pTrans = parent.transform;

		HingeJoint hj = parent.AddComponent<HingeJoint>();
		hj.axis = Vector3.up;

		JointMotor jm = hj.motor;
		jm.targetVelocity = 100.0f;
		jm.force = 100.0f;
		hj.motor = jm;

		Rigidbody rBody = parent.rigidbody;
		rBody.constraints = RigidbodyConstraints.FreezePosition | RigidbodyConstraints.FreezeRotationX | RigidbodyConstraints.FreezeRotationZ;

		GameObject obj, edge;
		Transform trans;
		Vector3 scale = new Vector3(leng, 1.0f, 1.0f);
		Vector3 eScale = new Vector3(1.0f, 0.5f, 1.0f);
		Vector3 ePos = new Vector3(leng / 2.0f, 0.0f, 0.0f);
		for (int i = 0; i < num; ++i)
		{
			obj = GameObject.CreatePrimitive(PrimitiveType.Cube);
			trans = obj.transform;
			trans.localScale = scale;

			edge = GameObject.CreatePrimitive(PrimitiveType.Cylinder);
			edge.transform.localScale = eScale;
			edge.transform.position = ePos;

			edge.transform.parent = trans;

			if (i != 0)
				trans.Rotate(0, angle * (float)i, 0);
			trans.Translate(pos, 0.0f, 0.0f);

			trans.parent = pTrans;
		}

		return;
	}
}

歯の数に応じて歯車の大きさが変わるようになってます。
歯の間隔は同じになるようにしているので、歯車の位置を調整するだけで動くはず。
あまり深く噛んでしまうと回らなくなるので、適度な位置にしてやるのがコツ。
実際に使ってみたテスト。
http://tumbling-earth.sakura.ne.jp/unity/watch/