Bonnet acts as a guidance and support to the stem. The thread area at top end of the bonnet in engage with stem thread. While rotating the fly wheel the stem thread will get screwed into to the bonnet thread and stem and disc in turn slide down to control the flow. There different types of bonnet assemblies are used depend on valve size. For small size globe valves integral bonnet construction can be used and for small size to moderate size threaded bonnet and for higher sized valves bolted bonnet can be used.