Usually, not always, the transfer bar is linked to the trigger and hammer blocks are linked to the safety.
Howdy
There is no safety in a revolver. (Yes, there was one S&W model that I am aware of that had a safety, but that was very unusual.) Hammer blocks are actuated by the rebound slide in a S&W revolver.
Let's go through a little bit of history. Originally S&W double action revolvers had no hammer block of any kind inside them. This is a photo of the lockwork of a 38 Military And Police revolver that shipped around 1908. With all double action revolvers the hammer must be withdrawn slightly when the trigger is released. This is so the firing pin does not remain extended into a spent primer, preventing the cylinder from swinging opening when desired. This is the mechanism that S&W adopted about 1905 for all of their double action revolvers. There was a slightly different mechanism from 1899 up until 1905. Anyway, the sliding piece behind the trigger is the Rebound Slide. There is a very strong coil spring inside it, which is partially visible in the slot at the rear of the part. When the trigger is released, the Rebound Slide spring pushes the trigger into the forward position as seen here. The bump on the top of the Rebound Slide wedges the bottom of the hammer up, causing it to retract slightly from the frame and retracting the firing pin back. This allows the cylinder to swing open when wanted. For a number of years, this was felt to be a good enough safety to keep the hammer and firing pin retracted, and a strong enough system that if the hammer were struck, the bump on the top of the Rebound Slide would prevent a round under the hammer from firing. However, over time it was found out that a strong enough blow could snap off the bottom of the hammer, where I drew the red line, or could crush the Rebound Slide (it is hollow after all) enough to allow a round under the hammer to fire.
This is a 38 M&P that left the factory in 1920. It has a hammer block inside. I am not sure exactly when S&W began installing this style hammer block, but I know this revolver left the factory in 1920. Sorry for the dirty rag for a backdrop, I was busy cleaning a lot of old goop out of this old revolver and took the opportunity to snap a photo.
This is the hammer block inside the 1920 38 M&P. This is the first of three different styles of hammer blocks that S&W has installed in their revolvers over the years. If you look closely at the photo above, you will see there is still a bump on top of the Rebound Slide forcing the hammer back. The hammer block is a piece of spring steel installed in a slot in the side plate of the revolver. It has a rectangular tab at the top which projects out towards us. Normally, with the Rebound Slide having forced the hammer slightly back, the tab will sit between the hammer and the frame. I have placed the hand in its position in the side plate for the photo. When the hand rises to rotate the cylinder, it wedges a pin forward, which in turn pulls the hammer block back into its slot, retracting the tab, so the hammer can fall all the way. Because the hammer block is made of spring steel, when the hand is pulled down, the hammer block will spring back into position blocking the hammer from moving all the way forward.
This is the second style of hammer block that S&W installed in their revolvers. Again, I do not know exactly when the change was made. I do know this 38 M&P shipped 1939. This is a simplified version of the earlier design. Again, the hammer block itself is a piece of spring steel, and a tab at the top extends forward into the space between the hammer and the frame. As can be seen in this photo, there is still a bump on top of the rebound slide that has forced the hammer back slightly, creating the space for the tab on the hammer block. I suspect S&W adopted this style of hammer block because it had less parts and would therefor be less expensive to produce than the earlier version. I have no proof of this, but S&W has always striven to drive the cost to manufacture out of their products, and less parts would mean less expense. There is a ramp on the hand. The ramp engages the horizontal tab, a new feature, on the side of the hammer block. When the hand rises, by engaging the horizontal tab, it shoves the hammer block deeper into its slot, withdrawing the upper tab from between the hammer and the frame, allowing the hammer to fall all the way.
This is the style of hammer block that failed in the incident in 1944 aboard a warship, killing a sailor. I have read as much as I can about this. I find no reference to how far the revolver fell, or it if was the sailor's revolver or somebody else's revolver. All that I have been able to find is that a 38 M&P, perhaps a Victory Model, fell to the deck of a warship and discharged, killing a sailor.
S&W had a large contract at the time, providing Victory Model revolvers to the Military. They were ordered by the government to come up with a fix if they wanted to retain their contract. S&W instituted a crash engineering program to get to the bottom of the problem, as well as come up with a new design. Tests showed that a revolver dropped from waist high could indeed discharge if it fell onto its hammer. The best explanation was that hardened cosmoline inside the revolver had prevented the hammer block from springing back to the 'safe position' blocking the hammer. This allowed the hammer to fall all the way, probably breaking the bottom of the hammer off, or perhaps crushing the rebound slide, allowing a round to discharge.
A new design was created within the space of one week.
This is the new design, and it is the same style of hammer block that has been inside every S&W revolver manufactured from 1944 up until today. The hammer block is the long, thin piece sitting at an angle. It rests in a slot in the side plate very similar to where the slot was for the older style hammer blocks. Like the older hammer blocks, there is a tab, hidden in this view, that extends from the surface of the hammer block to block the hammer. Yes, there is still a bump on top of the rebound slide to withdraw the hammer from a spent primer and create a space for the hammer block to rest in. I have placed the hammer block in the position it rests in in its slot in the side plate for this photo. There is a pin mounted in the rebound slide. When the hammer is cocked manually, or if the trigger is pulled double action, as the rebound slide moves back, the pin on the rebound slide pulls the hammer diagonally down, retracting it from the space between the hammer and the frame. When the rebound slide moves forward again, the hammer block slides up again to block the hammer. Yes, all S&W hammer blocks have always been a redundant safety device. Notice in this photo the hammer is not actually contacting the hammer block. It has been retracted enough so that there is space between it and the hammer block. All three S&W hammer blocks have always shared this design feature, they were always a redundant safety device, only put into play if the hammer or rebound slide were damaged. Unfortunately, the earlier design did not stand up to its design purpose when the poop hit the fan. By the way, back during WWII S&W had a recall of all the Victory Models in the field to install the new style of hammer block. Going forward, all the newly manufactured Victory Models had the new style hammer block inside, and instead of just having a V prefix in the serial number, the ones with the new hammer block had a SV prefix.
Transfer Bars:
This is a Ruger New Vaquero completely disassembled. Like all Ruger single action revolvers manufactured since some time in the mid 1970s, it has a transfer bar inside.
The Ruger frame is not conducive to showing the parts in position in the frame, so this is the best I can do to show the relationship between the parts. The transfer bar is the long thin part attached vertically to the trigger. When the hammer is cocked, the trigger spring pivots the trigger back, which also pushes the transfer bar up, positioning it between the hammer and the frame mounted firing pin. When the hammer falls, the transfer bar "transfers" the force of the hammer blow to the firing pin, discharging a round. When the trigger is released, it pulls the transfer bar back down away from the firing pin. In the 'at rest' position of the parts, the upper most surface of the hammer is in contact with the frame. The firing pin clears the recess below the top surface of the hammer, so the hammer cannot physically contact the firing pin.
The transfer bar never rises all the way to completely cover the firing pin. Only about half way, as show in this photo of a Vaquero with the hammer cocked and the transfer bar in the raised position. In order to cover the entire firing pin, the trigger would need more travel in order to raise the T bar that far.
There is a small spring loaded pin in the rear of the cylinder pin of a Ruger single action revolver, it is visible in the exploded view of the Vaquero above. The purpose of this spring is to push the transfer bar back as it rises. If for some reason the spring loaded pin does not push the transfer bar back as it rises, it can jam against the underside of the firing pin, preventing the hammer from being cocked all the way. I discovered this many years ago when heavy loads caused the cylinder pin on my old Blackhawk to jump forward out of engagement with its retaining latch. In this position the spring at the rear of the cylinder pin could not do its job of keeping the transfer bar from jamming under the firing pin, preventing the hammer from going to full cock. There are stronger aftermarket springs available for the cylinder pin latch to prevent this from happening.
Yes, transfer bars can break, rendering the revolver unable to fire, but generally this does not happen very often. I know a few CAS shooters who have broken transfer bars, but they often shoot zillions of rounds per year. Some even keep extra transfer bars handy in case they need one.
There is a condition called Transfer Bar Pinch which can cause this. Easily remedied by an experienced Ruger gunsmith, but not a problem in most cases.