My wife purchased this cart from an online vender for $90. On our first outing with it one of the wheels just fell off. To my surprise I found that although the outside of the wheel bearings had metal shields designed to protect them from dirt and debris, there were no shields on the inside and the pins holding the wheels on were weak and not secured well. It was pretty obvious to me that this cart was thrown together as cheaply as possible and not meant to last long. I therefore decided to simply replace all four wheels with better ones with solid tires (over the winter all four wheels had lost their air). This instructable is designed to show you how to do it with a minimum of time and hand tools - even if you are not the least bit mechanically inclined. There are a lot of pictures because frankly that is the easiest way to demonstrate things, even if they seem a bit redundant. The most difficult thing about this operation is that the wheels cost about $17 each, making four of them about as expensive as the original cost of the cart. Alternatively you can save money by replacing them one at a time as needed. You will need: 4 wheels, 5/8th inch washers (x8) and 4 new cotter pins or the equivalent. You'll also need a pair of pliers and gloves. No other tools are necessary.
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Step 1: The Old Wheels
The first picture shows the parts that went wrong. Each wheel has 2 bearings, one on the outside and one on the inside of the wheel hub. They are 'press fit' in place. Which means that a machine much stronger than me has pounded them into the ends of the hubs, essentially making it impossible for the average person to remove and replace the old bearings without destroying the hub and the wheel. The second picture shows the view through the original wheel hub looking at the inside face of the bearing at the far end. There is no metal covering (shield) over the ball bearings packed in grease. It is just a matter of time before a bearing like this exposed to outside dirt and grime will clog up and destroy the bearing. The third picture shows the inside of the bearing in the new wheel by comparison.
Step 2: The New Wheels
These are the replacement wheels I found at Harbor Freight. HaulMaster 10" 'Worry Free' Tires model #96691. They are the same outside diameter (10"), same width and have the same 5/8" bore as the originals. If you don't have a Harbor Freight store nearby, you can probably order them online. Make sure to order the correct size cotter pins from the next step at the same time. These wheels have a slightly longer length hub so we'll just have to remove the old spacer to make it fit. If you have a store nearby, take the old wheel with you. Inspect the wheels in the store and spin both bearings in the hub to make sure you got a good one. Although these are probably better wheels than the originals, they are still not exactly premium and you will need to inspect them occasionally and insure that they still spin well. Don't be tempted to use hard plastic lawn mower wheels, they aren't designed for a lot of weight and will crack under pressure. Some of them don't even have bearings in them.
Step 3: Remove the Old Wheels and Put the New Ones On
The wheels are held on by a pin (called a cotter pin) that goes through a hole at the very end of the axle. Just pull the pin out with some pliers and the wheel comes right off. You may need to first unbend the pin to remove it. Save the pin in case you have to use it again. Swap out the old spacer for a new 5/8" washer and put on the new wheel; the side of the hub that sticks out goes on first. Once the new wheel is in place put another 5/8" washer over the top of the axle and then install a new cotter pin to hold it all together. You can re-use the old washer if the new one is too thick and blocks the cotter pin hole. Once you've threaded the cotter pin through the axle, bend the pin over to secure it. Follow the directions in the pictures.
Step 4: Finish
This is a picture of the cart with pins properly securing the new wheels. I used the old washer on the front tire to get the cotter pin to fit. The old wheels are in the foreground. My guess is these principles are probably the same for a lot of wagons and carts so think about fixing it yourself before you throw it away.