Strength equipment manufacturers provide a range of machines for building the crushing grip. These can be used to develop overall hand strength in a progressive manner. Performing exercises such as negatives is greatly simplified. Some grip machines even offer features such as filed handles for beyond the range training.
Specificity of training does require that one practice applying strength developed on grip machines to grippers before carry over will be seen. A grip machine is not a necessity to build a strong crushing grip and close hard grippers.